370 THE EVOLUTION OF LIFE. 



the second system of external blood-vessels is, to the im- 

 placental embryo, of no greater avail than the first ; and 

 since the communication between the embryo and the 

 placenta among placental mammals, might as well or better 

 have been made directly, instead of by metamorphosis of 

 the allantois ; these substitutions appear unaccountable as 

 residts of design. But they are quite congruous with tho 

 supposition, that the mammalian type arose out of lower 

 vertebrate types. For in such case, the mammalian embryo, 

 passing through states representing, more or less distinctly, 

 those which its remote ancestors had in common with the 

 lower Fertehrata, develops these subsidiary organs in like 

 ways with the lower Fertebrata. 



Even more striking than the substitutions of organs are 

 the suppressions of organs. Mr Darwin names some cases 

 as " extremely curious ; for instance, the presence of teeth 

 in foGtal whales, which when grown up have not a tooth in 

 their heads ; * * * It has even been stated on good 

 authority that rudiments of teeth can be detected in the 

 beaks of certain embryonic birds.'* Not even temporary 

 functions can be assigned for these organs that are first 

 built up and then pulled down again. They are absolutely 

 useless — ^their formation is absolutely superfluous. Irrecon- 

 cilable with any teleological theory, they do not even har- 

 monize with the theory of fixed types which are maintained 

 by the development of all the typical parts, even where not 

 wanted; seeing that the disappearance of these incipient 

 organs during foetal life, spoils the typical resemblance. 

 But while to all other hypotheses these facts are stumbling- 

 blocks, they yield strong support to the hypothesis of evohi- 

 tion. 



AUied to these cases, are the cases of what has been called 

 retrograde development. Many parasitic creatures and 

 creatures which, after leading active lives for a time, eventu- 

 ally become fixed, lose, in their adult states, the limbs and 

 senses which they had when young. It may be allegodi 



