STRUCTURE AND DEVELOPMENT 69 



vertebrates. It was difficult perhaps to believe that 

 these successive grades of organic structure indicated 

 an order of evolution, because it seemed impossible 

 that an animal so simple as a protozoan could produce 

 offspring with the complex organization of a frog or a 

 cat, even in long ages. But development delivers its 

 evidence relating to this matter with telling and im- 

 pressive force. How can we doubt the possibility of 

 an evolution of higher animals from ancestors as simple 

 as Hydra and Amoeba when a frog and a cat, like all 

 other complicated organisms, begin individual existence 

 as single cells, and pass through gastrula stages ? If we 

 deny it, we contradict the evidence of our senses, for 

 the development is actually accomplished by the trans- 

 formation of a single cell into a double-walled sac, and 

 of this into different and more intricate organic mechan- 

 isms. The process can take place, for it does take place. 

 Not until the investigator becomes familiar with a wide 

 range of diverse animals and the peculiar qualities of 

 their similar early stages, can he estimate the tremen- 

 dous weight of the facts of comparative embryology. 

 Were the statement iterated and reiterated on every 

 page and in every paragraph, there would be no undue 

 emphasis put upon the astounding fact that the appar- 

 ently impassable gap between a one-celled animal like 

 Amoeba and a mammal like a cat is actually compassed 

 during the development of the last-named organisms 

 from single cells. The occurrence of gill-slits in the 

 embryos of lizards, birds, and mammals now seems a 

 small thing when compared with the correspondences 

 disclosed by the earliest stages of development. But 

 in spite of their complexity, all the changes of " growing 



