THEORY OF EVOLUTION 501 



different subdivisions in relation to the particular habits of life ; still 

 they remain fundamentally alike because they have developed from 

 the underlying plan of organization found in the ancestors. The 

 seal and the bird, although quite different, show similarities in habits 

 and otherwise because of common ancestry long ago. The conclu- 

 sion of biologists of today is that all of the animals in a group, such 

 as the vertebrates, have arisen by descent with change from a primi- 

 tive organism which possessed the fundamental organization as shown 

 from cyclostomes to man. 



Embryological Evidence. — Evidences from this field really con- 

 tinue directly from the previous discussion. The animal is to be 

 thought of as an individual from the single-celled zygote stage to 

 the mature stage of old age, no matter what its complexity is. 

 Intimately related types of animals parallel through a large portion 

 of their development to diverge somewhat in adult condition, more 

 remotely related forms take separate developmental courses rather 

 early in life, and unrelated forms may be different almost from the 

 beginning. In numerous instances the developing stages of more 

 advanced forms resemble very closely the mature stages of the less 

 advanced types in a serial fashion. The history of the individual 

 animal often corresponds in a general way to the history of the 

 advances of the animal kingdom, up to its state of development. 

 This apparent repetition of the ancestral development in individuals 

 was what led Haeckel to formulate the recapitulation theory, ex- 

 pressed briefly: ontogeny repeats or recapitulates phylogeny, as has 

 been discussed earlier in the chapter. 



In the vertebrate group these apparent relations are much shown. 

 Most of the embryos of this group are so similar that it is nearly 

 impossible to distinguish them. They pass through the identical 

 stages of development. Systems, such as the circulatory, nervous, 

 digestive, and respiratory, follow the same course of development in 

 all of the vertebrates, no matter how simple or complex. In earlier 

 stages the similarity is strikingly close. 



The course of development and the modifications shown in the 

 aortic arches or main arteries leaving the heart and passing through 

 the gill regions is a specific example of the manner in which a spe- 

 cific set of structures follows out a repetition of ancestral stages in 

 the development of the individual. Fig. 204 show^s a comparison 

 of the arrangement of the branchial (aortic) arches from the primi- 



