XXI.] EMBRYOLOGY. 261 



been independently created, and which are explicable only 

 as records of the descent of the organisms that possess them 

 from ancestors to which they were of use. And here, in 

 embryology, exactly the same confirmation comes in. 

 There are not only useless organs, but useless modes of useless 

 development, such as the formation in the embryos of air- dlvelop*- 

 breathing Vertebrates, of " branchial slits " and branchial ™ent. 

 circulation, which afterwards disappear and never perform 

 any function. This is an exactly parallel case to that of 

 rudimentary organs ; and some of what are strictly rudi- 

 mentary organs are found in embryos, and disappear before Eudi- 

 the organism comes to its mature state. Thus the embryo ^^'^^''^'T 



° •' organs in 



of the Greenland whale has teeth, which disappear by the the em- 

 time it is fully grown ; the calf has certain teeth before ^^° °^ ^' 

 birth which are afterwards absorbed;^ and the young of 

 some of the naked " nudibranchiate " MoUusca have dis- 

 cernible shells.- These, like the wing-bones of the wing- 

 less apteryx, are only intelligible as records of organs 

 which were developed in the ancestors of those species, 

 and were of use to them. It is also a significant fact, that Rudi- 

 rudimentary organs, even when they remain through life, ™gaiiT ^ 

 are relatively smaller in the mature form than in the largest 

 embryo ; ^ this is no doubt a case of the law that unused embryo, 

 organs diminish in size. 



As I have stated at the beginning of this chapter, all Differen- 

 organisms are developed from germs which are perfectly gJ^^'^Qg 

 ahke ; and they grow more and more unlike each other 

 as their development proceeds. This may be called the 

 differentiation of embryos one from the other. It is now 

 time to state the law according to which that differentiation 

 takes place. 



It is a familiar truth, that organic species are naturally Groups of 

 arranged in gi'oups, and these groups, again, in wider S''°"P^- 

 groups. Thus, for instance, the domestic fowl is one of 

 the group of birds, and birds are a part of the wider group 

 of Vertebrates. The law in question is, that as tlie deve- 



1 Darwin's Origin of Species, p. 534. 



2 Carpenter's Comparative Physiology, p. 318. 

 * Darwin's Origin of Species, p. 537, 



