Facts and Arguments for Darwin Informative Summary


“Facts and Arguments for Darwin” is a work by Fritz Müller, a German naturalist who studied crustaceans in Brazil. The book is a collection of observations and arguments in favor of Darwin’s theory of evolution by natural selection. Müller’s work is notable for its detailed and insightful examination of crustacean development, including their various larval forms. He argues that the diversity of crustaceans can be explained through a process of gradual evolution, with various lineages diverging from a common ancestor.

Müller’s book is a powerful testament to the importance of detailed observation in scientific discovery. By meticulously studying the anatomy and development of various crustacean species, he was able to uncover patterns and relationships that supported Darwin’s theory. His work helped to solidify the acceptance of evolution among the scientific community, providing a strong foundation for further research in evolutionary biology.

Key Findings:

  • The absence of contradictions in applying Darwinian theory to a specific group (crustaceans): Müller meticulously searched for contradictions within the theory when applied to his field of study, but found none. This lack of contradictions strengthened the credibility of Darwin’s theory.
  • Early Naupliiform larvae of shrimp demonstrate a common ancestor: Müller discovered Nauplius-like larvae of shrimps, suggesting that even higher crustaceans like shrimps once passed through this larval stage, a critical finding for understanding evolutionary relationships.
  • Tanais and Orchestia showcase dimorphism in males: These crustaceans exhibit two distinct male forms with different characteristics, suggesting that variations in sexual traits can drive evolutionary divergence.
  • Air-breathing crabs have diverse adaptation mechanisms: Müller’s observations of land crabs revealed varying adaptations for aerial respiration, highlighting how different evolutionary pressures can lead to unique solutions in related groups.
  • The structure of the heart varies within closely related orders: The heart of the Amphipoda is uniform, while that of the Isopoda is highly variable, illustrating the impact of evolutionary pressures on internal organs.
  • Developmental history of crustaceans reveals common ancestry: Müller’s detailed account of crustacean development reveals a pattern of gradual evolution, with various forms emerging from a shared ancestral stage.


  • Evolutionary theory is supported by detailed observation: Müller’s book emphasizes the importance of meticulous observation and the power of Darwin’s theory in providing a framework for understanding biological diversity.
  • The diversity of life results from gradual changes over time: By studying crustaceans, Müller demonstrates how natural selection can drive subtle changes in populations over generations, ultimately leading to the emergence of new species and lineages.
  • Development reflects evolutionary history: The developmental stages of organisms often provide a glimpse into their evolutionary history, as they may recapitulate features of their ancestors.
  • Adaptations are not always perfect, but they are functional: Müller’s observations of air-breathing crabs showcase how different groups can adapt to similar environments in unique ways, illustrating the non-teleological nature of evolution.
  • Sexual selection can drive rapid evolutionary change: Dimorphism in male crustaceans, as exemplified by Tanais and Orchestia, illustrates the role of sexual selection in shaping evolutionary change, particularly in reproductive traits.
  • Developmental processes can be both simplified and complexified through evolution: The transition from complex larval stages to direct development, as seen in some crustaceans, highlights the potential for both simplification and complexification of development over evolutionary time.

Historical Context:

The book was published in 1869, a period of intense debate surrounding Darwin’s theory of evolution. The scientific community was still grappling with the implications of Darwin’s ideas, with some embracing them wholeheartedly while others remained staunchly opposed. Müller’s work, with its wealth of detailed observations and insightful arguments, provided compelling evidence for Darwin’s theory and contributed significantly to its growing acceptance.


  1. Many crabs breathe air using a posterior opening: These crabs have adapted to life outside of the water by developing a separate opening in their branchial cavity for air intake. This adaptation is found in various families of crabs, including the Grapsoidae, Ocypodidae, and Eriphinæ.
  2. The calling crab (Gelasimus) does not always beckon with its claw: While the male calling crab often holds its large claw up, not all species do this, indicating variations within the genus.
  3. The olfactory filaments of crustaceans may be associated with smell: Male crustaceans often have more olfactory filaments on their antennae, suggesting a role in attracting females.
  4. Tanais males have two distinct forms: The males of this crustacean species exhibit two distinct forms, one with large chelae and the other with many olfactory filaments on its antennae.
  5. The heart of the Amphipoda is consistent in structure: This order of crustaceans possesses a heart with three pairs of fissures in a specific position, demonstrating a conserved structure.
  6. The heart of the Isopoda is highly variable: In contrast to the Amphipoda, the heart of the Isopoda exhibits a wide range of structures and positions, demonstrating evolutionary divergence.
  7. The parasitic Isopods (Bopyridae) exhibit extensive retrograde metamorphosis: These Isopods, which live on crabs and lobsters, have lost many of their typical features, such as eyes, antennæ, and limbs, due to their parasitic lifestyle.
  8. Amphipoda embryos are curved downwards: This characteristic differentiates Amphipoda embryos from those of Isopods, which are curved upwards.
  9. The micropylar apparatus in Amphipoda is a temporary structure: This unique structure, which attaches the Amphipod embryo to the egg membrane, disappears soon after hatching.
  10. Some Amphipoda species show significant sexual dimorphism: The differences between male and female Amphipoda can be so pronounced that they were once considered different species.
  11. The developmental history of some Prawns (Penëus) involves Nauplii, Zoëæ, and Mysis-like stages: This process showcases the gradual transformation of a Nauplius larva into a Prawn, reflecting its evolutionary history.
  12. The Zoëæ of crabs typically have long, spiniform processes on their carapace: These processes provide protection for the young crabs, highlighting the adaptive nature of larval structures.
  13. The Zoëæ of Hermit Crabs have a well-developed antennal scale: This structure is homologous to the antennal scale of adult Prawns and is present in the Zoëæ of Hermit Crabs but not in adults.
  14. The caudal feet of Euphausia and Prawns with Nauplius brood form freely on the ventral surface: This contrasts with Prawns with Zoëa brood, Pagurus, and Porcellana, where the caudal feet form within the caudal lamellæ.
  15. The chelae on the penultimate pair of feet in young Brachyscelus are an acquired adaptation: These chelae aid in the adhesion of the young Brachyscelus to their host, showcasing the adaptive nature of larval structures.
  16. The Zoëæ of various Decapods, including crabs, share common characteristics: This suggests a shared ancestry and a common Zoëa stage in their evolutionary history.
  17. The spinous processes on the carapace of Zoëæ are likely adaptations acquired through natural selection: These processes protect the young crabs from predation, demonstrating the role of natural selection in shaping larval forms.
  18. The roots of the Rhizocephala are homologous to the cement ducts of the Cirripedia: This finding suggests that the Rhizocephala evolved from Cirripedes, with the cement ducts evolving into nutritive roots.
  19. Cirripedes exhibit a diverse range of hosts, including sponges, corals, and crustaceans: This diversity suggests that they may have evolved to exploit a variety of niches, leading to the emergence of the Rhizocephala.
  20. Anelasma squalicola, a Lepadide that lives on Sharks, has lost its cirri and buccal organs: This demonstrates the potential for a parasite to adapt to a new food source and reduce its reliance on its original feeding apparatus.


  1. 60 Brazilian Amphipoda species discovered by Müller: This contrasts with 28 species from Arctic seas, highlighting the potential diversity of Amphipoda in different regions.
  2. Hundreds to thousands of Tanais per square inch of Confervæ: This demonstrates the high density of this crustacean species, highlighting the potential for rapid evolution due to intense competition.
  3. The abdomen of a Porcellana Zoëa is 8 times longer with its spinous processes: This illustrates the significant impact of these processes on size and the need for a larger predator to swallow the young crab.
  4. 11 abdominal segments in Orthoptera: This number of segments aligns with the Malacostraca, suggesting a possible evolutionary relationship.
  5. Thousands of species in the Polypodiaceæ family of ferns: This contrasts with the few species in the Schizæaceæ family, showcasing the varied levels of diversity within related groups of organisms.
  6. The heart of the Calanidæ and Pontellidæ Copepoda possesses a heart, unlike the Cyclopidæ and Corycæidæ: This highlights the variation in heart structure within a single order, demonstrating evolutionary divergence.
  7. The carapace of a Cirripede can be overleaped during development: This is exemplified by Cryptophialus minutus, where the pupa stage directly follows the egg-like larva, skipping the Nauplius stage.
  8. Six pairs of natatory feet on the abdomen of the pupae of Cirripedia and Rhizocephala: This similarity in structure highlights their close evolutionary relationship.
  9. Three club-shaped horns on the egg of Tetraclita porosa: This unique feature of the egg demonstrates the diverse morphology of even early developmental stages.
  10. Thousands of years of evolution reflected in a few weeks of development: This highlights the potential for developmental history to provide insights into the evolutionary history of a species.
  11. The opercular peduncle of Serpula is homologous to a branchial filament: This highlights the developmental stages as a tool for understanding homologous structures.
  12. Hundreds of volunteers and donations sustain the Project Gutenberg mission: This highlights the crucial role of collaboration and public support in promoting free access to knowledge.


  1. Natural selection: The process by which organisms with advantageous traits survive and reproduce more successfully than those with less advantageous traits, leading to gradual evolutionary change.
  2. Zoëa: A larval stage found in many higher crustaceans, characterized by a carapace with spinous processes, a lack of abdominal appendages, and a particular type of locomotion.
  3. Nauplius: A larval stage found in many lower crustaceans, characterized by an unsegmented body, three pairs of limbs, and a median eye.
  4. Dimorphism: The existence of two distinct forms within a species, usually related to sex.
  5. Abdomen: The posterior region of a crustacean body, often bearing appendages for swimming, respiration, or reproduction.
  6. Carapace: A hard, protective covering that protects the cephalothorax of crustaceans.
  7. Maxillipedes: A type of appendage used for feeding, often modified for manipulation and transport of food to the mouth.
  8. Brood-pouch: A structure in some crustaceans where eggs are protected and the young are developed.
  9. Segmentation: The division of the body into repeating segments, a common feature of arthropods.
  10. Retrograde metamorphosis: A type of development where an organism loses or simplifies features in its adult form compared to its larval form.


  1. The evolution of air-breathing crabs: Müller’s observations of different crab species, such as Aratus Pisonii, Sesarma, Cyclograpsus, and Ocypoda, demonstrate the diversity of adaptations for aerial respiration in land crabs.
  2. The two forms of males in Tanais: Müller meticulously documented the two distinct male forms of this crustacean, highlighting the role of sexual selection in creating variation within a species.
  3. The development of the Prawn with Nauplius brood: Müller traced the development of this prawn from a Nauplius to its adult form, demonstrating the gradual evolution of complex features from simpler ones.
  4. The development of the Tubicolar Annelids: Müller observed the development of these worms, showcasing the gradual emergence of specialized features, such as the operculum, in a lineage.
  5. The parasitic Isopods (Bopyridae): These Isopods demonstrate the extreme evolutionary adaptation of parasitic organisms, showing how they can lose complex structures and rely on their host for sustenance.
  6. The development of the Amphipoda: The “micropylar apparatus” in Amphipoda embryos, which helps with attachment to the egg membrane, is a unique example of a temporary structure that disappears after hatching.
  7. The development of Hyperia: This Amphipod species illustrates how larval stages can be adapted for a different lifestyle than the adult, showcasing the evolutionary pressures on different developmental stages.
  8. The development of Brachyscelus: This crustacean illustrates how a larva can acquire structures, such as the chelae on its penultimate pair of feet, that are not present in the adult form.
  9. The Rhizocephala: This group of parasites, which are derived from Cirripedes, demonstrates the potential for dramatic evolutionary change, including the loss of mouthparts and the development of nutritive roots.
  10. Anelasma squalicola: This Lepadide, which lives on sharks, illustrates the potential for a parasitic organism to evolve a new food source and reduce its reliance on its original feeding apparatus.


Fritz Müller’s “Facts and Arguments for Darwin” provides compelling evidence for Darwin’s theory of evolution through detailed observations of crustacean anatomy and development. Müller’s work highlights the importance of detailed observation in scientific discovery and demonstrates how natural selection can drive gradual change over time. By tracing the development of various crustaceans, he reveals a pattern of evolutionary relationships, illustrating how different lineages can diverge from a common ancestor. Müller’s book remains a valuable resource for understanding the evolutionary history of crustaceans and serves as a testament to the power of Darwin’s theory in explaining the diversity of life.

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