MODIFICATION OF SPECIES 



351 



Similarities in physiological properties are quite as abundant as are 

 likenesses of structure. The enzymes of digestion are in general very 

 much alike in different vertebrate animals. As a rule, protein-spJitting 

 enzymes are produced and used in corresponding organs in different 

 ^'ertebrates. Nervous and hormone control are in most respects alike. 

 Even the composition of the blood shows close similarity between animals 

 whose structures are alike; the hemoglobin (page 127) has nearly the same 

 crystalline chai-acters, and the serum has almost the same chemical com- 

 position as shown by precipitin tests. In using this precipitin reaction 

 an animal is rendered immune to, let us say, sheep blood by repeated 

 injection of sheep blood into its veins. This immune blood then pro- 



FiG. 299. — Sacculina, parasitic on crabs. A, young Sacculina, shortly after hatching. 

 B, young animal shown attached to its host, the crab. The projection at the anterior end 

 has penetrated the chitinous ventral wall of the abdomen of the crab, only a small piece of 

 the chitin being shown. C, adult Sacculina (s), consisting of a pulpy mass on the under 

 side of the crab's abdomen, and a host of branching processes in the host's body. A and 

 B greatly but unequally magnified, C reduced. 



duces a white precipitate when blood of a sheep or of an animal very 

 similar to sheep is mixed with it, but not when blood of a very different 

 sort of animal is mixed with it. The precipitate is formed only in 

 response to blood of a given chemical composition, and similar composi- 

 tion has been found almost solely in the blood of animals that are 

 structurally similar. 



The argument from all these similarities, already advanced on page 255, 

 is that only heredity — hence common ancestry — could account for them. 

 But if two species of animals have come from a common source, any 

 differences between them — and there always are differences — must have 

 arisen 'since the time of the common ancestors. Emphasis is now to be 

 put on these differences, for their origin constitutes evolution. 



The other principal evidences of evolution are derived from fossils, 



