THE ARGUMENTS FROM EMBRYOLOGY. 457 



to the natural-history student is that IJie tadpole breathes 

 by external branchiae, and that these, needful during its 

 aquatic life, dwindle away as fast as it develops the lungs 

 fitting it for terrestrial life. But in one of the higher 

 Amphibia, the viviparous Salamander, these transformations 

 ordinarily undergone during the free life of the larva, are 

 undergone by the embryo in the egg. The branchiae are 

 developed though there is no use for them : lungs being sub- 

 stituted as breathing appliances before the creature is born. 



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 

 foetal 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." Irreconcilable with any teleological theory, 

 these facts do not even harmonize with the theory of fixed 

 types which are maintained by the development of all the 

 typical parts, even where not wanted; seeing that the dis- 

 appearance of these incipient organs during fcetal life spoils 

 the typical resemblance. But while to other hypotheses these 

 facts are stumbling-blocks, they yield strong support to the 

 hypothesis of evolution. 



Allied 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, become 

 fixed, lose, in their adult states, the limbs and senses they 

 had when young. It may be alleged, however, that these 

 creatures could not secure the habitats needful for them, 

 without possessing, during their larval stages, eyes and swim- 

 ming appendages which eventually become useless; that 

 though, by losing these, their organization retrogresses in one 

 direction, it progresses in another direction; and that, there- 

 fore, they do not exhibit the needless development of a 

 higher type on the way to a lower type. Nevertheless there 

 are instances of a descent in organization, following an appa- 



