564 APPLIED BIOLOGY 



We cannot explain the presence of a rudimentary organ 

 (such as an Apteryx wing, or whale hind limb) except as an 

 adaptation or change of a corresponding organ in closely 

 similar animals. Hence the Apteryx must have descended 

 from birds with wings, and whales from mammals with hind 

 legs. Such is the line of interpretation which biologists now 

 apply everywhere in the animal and plant kingdoms. 



495. Embryological Evidences of Evolution. In 364 

 we have noted that gill-slits appear in the embryos of all 

 vertebrates, although only fishes and young amphibians make 

 use of them. The appearance of embryonic organs which 

 never develop is very common in every group of animals, and 

 also in plants. The case of gill-slits is only one of many such 

 cases among higher vertebrates. 



For such appearance of useless embryonic organs there is 

 only one satisfactory explanation; namely, that they are 

 evidences of ancestral history. Zoologists now agree in believ- 

 ing that the presence of gill-slits in embryos of all verte- 

 brates means that all vertebrates were derived from fish- 

 like ancestors which needed gill-slits for respiration. But 

 why gill-slits should still persist in the embryology of every 

 vertebrate is a mystery. We can simply point to the facts 

 indicating that it is inherited from an ancestral condition, 

 an heirloom from the long-past history of the race of back- 

 boned animals. 



In still another way does the embryology of animals sug- 

 gest evolution from common ancestors, and that is in the 

 similar development. In formation of egg-cells and sperm- 

 cells, in fertilization of eggs, in cell-division, and in formation 

 of organs there is a surprising similarity among animals ; and 

 m case of animals whose structure shows close relationship 

 the similarity extends into great detail. Such facts lead to 

 only one interpretation; namely, that all animals have been 

 leveloped from the first forms of life. The facts from embry- 

 ology are so convincing as to relationship that classification 



