GROUNDS AND CONSEQUENCES OF EVOLUTION. 345 



differentiations, the complications and special adaptations 

 of the higher vertebrate come into existence. 



But what of all this? Very much indeed. This mar- 

 velous evolution which we see the higher vertebrate pass 

 through is absolutely identical with the embryonic history 

 of every other animal down to a certain point in its de- 

 velopment. Every animal begins in the egg, and the eggs 

 of all animals (we exclude shell and other accessories) are 

 completely undistinguishable in structure. Every animal, 

 except some of the very lowest, presents us, in its de- 

 velopment, the morula stage. Every animal, with a few 

 additional exceptions, passes also through the planula and 

 the gastrula stages. Thus every vertebrated animal pre- 

 sents us the same primitive furrow, the same cerebral 

 enlargements, the same segmentation, the same caudal 

 continuation, the same vascular area, the same one-cham- 

 bered heart, the same branchial arches and blood-vessels, 

 the same progressive changes in the development of the 

 brain, the same mode of formation of the enteric and 

 abdominal cavities, the same beginnings of the formation 

 of the face. This identity in embryonic histories may be 

 unexpected; it may be amazing; it may be humiliating; 

 but there is nothing better established in science.* 



This is not all. There are living creatures which rep- 

 resent these successive stages of embryonic development. 

 There are some so low that they never pass beyond the 



* The reader will find the subject discussed in E. Haeckel's Naturliche Schopf- 

 ungsgeschichte, xi Vortrag, and Anthropogenie, xiii-xix Vortrage (translated 

 and republished in New York as Natural History of Creation and The Origin of 

 Man); A. KSlliker, Entwickelungsgeschichte des Menschen u.der hoheren Thiere, 

 2d ed., Leipzig, 1876; P. M. Balfour, A treatise on Comparative Embryology, 

 London, 1880. A general synopsis in A. S. Packard's Life-histories of Animals, 

 including Man, New York, 1876; and a particular account of the history of the 

 chick, in M. Foster and F. M. Balfour, The Elements of Embryology, Part I. 



