366 ANIMAL BIOLOGY 



which at first are nearly indistinguishable from the conditions 

 which exist in the lowest Vertebrates. Then the changes become 

 progressively more complex along lines broadly similar to those 

 occurring from Fish to Mammal, until finally the complex human 

 brain is formed. (Figs. 140, 141.) 



The same picture is presented by a study of the development of 

 the excretory system, the reproductive system, the skull, and so 

 on. One cannot avoid the fact that the organs of higher animals 

 during development pass through stages which correspond with 

 the larval or adult condition of similar organs in lower forms. The 

 correspondence is far from exact — to be sure, there are gaps and 

 blurs — but it is not an exaggeration to say that embryological 

 development is parallel to that which anatomical study leads us 

 to expect. A knowledge of the anatomy of an animal actually 

 gives a sound basis of facts from which to predict in broad out- 

 lines its embryological development. (Figs. 129, 235.) 



What are the bearings of these facts on the evolution theory? 

 It is perfectly logical to conclude that it is an architectural neces- 

 sity, let us say, for the four-chambered heart to arise from a two- 

 and three-chambered condition - - and undoubtedly if this were 

 the only example of ' ontogeny repeating phylogeny ' the conclusion 

 would be justified. But when one considers the widespread general 

 correspondence of the developmental stages in higher forms with 

 conditions as they exist in the adults of lower forms, the facts al- 

 most overwhelmingly force us to go further and conclude that the 

 similarity has its basis in inheritance, in actual blood relationship 

 between the higher and lower forms, in descent with change — evo- 

 lution. 



5. Physiology 



Fundamental structural similarities throughout a series of or- 

 ganisms implies fundamental physiological similarities — struc- 

 ture and function go hand in hand, each being an expression of 

 the other: function alone gives permanence to structure — and 

 this is strikingly corroborated by the data accumulated which 

 show that species are characterized not only by morphological 

 attributes, but by their specific biochemical constitution as well. 

 But this physiological evidence of the relationships of organisms 

 is less readily presented in brief form, so we may confine attention 

 to the most significant examples. 



