LIFE HISTORY STUDIES OP ANIMALS. 505 



and simple — a thing which is common in physics, but very rare in biol- 

 ogy. I diverge from him when he says that " each animal is compelled 

 to discover its parentage in its own development," that " every animal 

 in its own development repeats this history, and climbs up its own 

 genealogical tree." When he declares that " the proof of the theory 

 depends chiefly on its universal applicability to all animals, whether 

 high or low in the zoological scale, and to all their parts and organs," ^ 

 I feel persuaded that, if this is really so, the recapitulation theory will 

 never be proved at all. The development, so far as it has yet been 

 traced, of a Hydra, Feripatus, beetle, pond mussel, squid, Amphioxus, 

 chick, or mammal, tells us very little indeed of the history of the races 

 to which they belong. Development tells us something, I admit, and 

 that something is welcome, but it gives no answer at all to most of the 

 questions that we put. The development of a mammal, for instance, 

 brings to light what I take to be clear proof of a piscine stage; but the 

 stage or stages immediately previous can only be vaguely described as 

 vertebrate, and when we go back farther still all resemblance to par- 

 ticular adult animals is lost. Th(i best facts of the recapitulationist 

 are striking and valuable, but they are much rarer than the thorough- 

 going recapitulationist admits; he has picked out all the big strawber- 

 ries, and put them at the top of the basket. I admit no sort of neces- 

 sity for the recapitulation of the events of the phylogeny in the devel- 

 opment of the individual. Whenever any biologist brings the word 

 ''must" into his statement of the operations of living nature I look out 

 to see whether he will not shortly fall into trouble. 



This hasty review of animal transformation reminds me how great is 

 the part of adaptation in nature. To many naturalists the study of 

 adaptations is the popular and superficial side of things; that which 

 they take to be truly scientific is some kind of index making. But we 

 should recognize that comparatively modern adaptations may be of 

 vital importance to the species, and particularly luminous to the stu- 

 dent because at times they show us nature at work. 



I am accustomed to refer such adaptations to the jDrocess of natural 

 selection, though if. anyone claimed to explain them by another process 

 I should, for present purposes, cheerfully adopt a more neutral phrase. 

 There are, I believe, no limits to be assigned to the action of natural 

 selection upon living plants and animals. Natural selection can act 

 upon the egg, the embryo, the larva, and the resting pupa as well as 

 upon the adult capable of propagation. It can even influence the race 

 through individuals which are not in the line of descent at all, such as 

 adults past bearing or the neuters of a colony. The distinction 

 between historical and adaptive, palingenetic and ccenogenetic, is rela- 

 tive only, a difference not of kind but of degree. All features are 



' The quotations are from the late Prof. A. Milnes Marshall's address to Section D, 

 Brit. Assoc. Rep., 1890, which states the recapitulationist case with great knowledge 

 and skill. 



