EVIDENCE FROM EMBRYOLOGY. 157 



These are the hind limbs which have become rudimentary, 

 and which do not subserve locomotion but, in the male at least, assist 

 in copulation. The blind worms possess a rudimentary shoulder 

 girdle and breast bone, although the anterior extremities are want- 

 ing : these bones may be connected with the need of protecting the 

 heart, or may aid in respiration. When we see that the upper 

 incisor teeth are developed in the foetus of many ruminants, and that 

 these teeth are never cut, and that the embryos of the whalebone 

 whales have the rudiments of teeth in their jaws, which they soon 

 lose and never make use of in mastication, it is much more rational 

 to ascribe to these .structures a part-in the growth of the jaw than to 

 hold them for wholly useless. The rudimentary wings of the penguin 

 are employed as oars, those of the ostrich as aids to running and as 

 weapons for protection. The rudimentary stumps of the kiwi, on 

 the contrary, appear valueless. In many cases we are not in a 

 position to assign any function or value to rudimentary organs. 



Evidence from Embryology. The results of embryology too, i.e., 

 the individual development from the ovum to the fully developed 

 form, are in complete agreement with the Darwinian theories of 

 selection and descent. The fact that the animals belonging to one 

 type have, as a rule, embryos which are much alike and undergo a 

 similar developmental process, and that the closer the relationship 

 between the adult forms the greater the similarity in their develop- 

 ment (with some remarkable exceptions), supports the conception of 

 a common ancestry and the hypothesis of differing gradations of blood- 

 relationship. 



If the groups of different value which correspond to the divisions 

 and subdivisions of our classification are genetically derived from 

 more or less remote ancestral forms, then the individual develop- 

 ment will present >o many the more common features the closer the 

 forms stand to their common ancestor. 



The fact that animals which differ much from one another and 

 exist under very different conditions of life show an unusual agree- 

 ment in their post-embryonic development up to a more or less late 

 period (the free Copepoda, parasitic Crustacea, Cirripedia), is in no wise 

 opposed to the theory, but may be explained by the influence which 

 adaptation has exerted not only during the period of sexual life, 

 but also during each developmental period, causing changes which 

 have been inherited in corresponding periods of life. 



The phenomena of metamorphosis afford numerous proofs of the 

 fact that the adaptation of the embryonic form is as complete as 



