142 PROBLEMS OF EVOLUTION 



a series of definite phases, at the end of the prescribed course 

 were are liberty to break out into wild haphazard developments. 

 Law and order so perfect surely never ended in chaos. There 

 is freedom to vary, but freedom within limits. Nevertheless, 

 even if variation were chaotic, if any animal might produce any 

 kind of organ, even then, given an unlimited number of in- 

 dividuals in a species, mere coincidence might, it is imaginable, 

 produce a sufficient number of representatives of some new type 

 to form a new species. But the great results achieved by 

 breeders of cattle, horses, pigeons, and other animals, have been 

 attained in many cases with quite small numbers. When breed- 

 ing for some particular point they have generally been able to 

 find more than one among the few at their disposal in which the 

 required tendency was traceable. The smallness of their re- 

 sources and the greatness of their achievements drive us to the 

 conclusion that variation is within certain limits. 



Con- To pass on to other evidence of limitation, we find among 

 evolution ev l u ^ omsts a growing tendency towards the view that species 

 and even larger groups not seldom tend to converge. Like 

 forms arise independently. There is reason to believe that in 

 the ostrich and its allies we have instances of convergent evolu- 

 tion : they are probably polyphyletic in origin, and have drawn 

 towards one another. The dentition of the marsupial animals, 

 according to their food and habits, approximates to that of 

 placental mammals of similar habits. The heart of a bird with 

 its four chambers is strangely like the heart of a mammal and 

 the difference of the avian and mammalian valves between the 

 two right-hand chambers only serves to bring out the similarity 

 of the main architectural features. In the Aye- Aye (Chiroinys) 

 the teeth completely simulate those of a rodent. 1 In this con- 

 nection much might be said on the subject of what Darwin called 

 analogous variation. 2 In all cases where we see convergent 

 evolution or analogous variation, we must assume that the two 

 groups before they parted company had reached a stage of 



1 See Huxley, Man's Place in Nature, p. 84. 



2 Animals and Plants under Domestication, vol. ii. pp. 348-352. See also Eimer's 

 Zoologische Studien auf Capri, 



