PREFORMATION AND EPIGENESIS 433 



we but throw the problem one stage farther back, and, as Weismann 

 himself has pointed out,^ substitute for one difficulty another of 

 exactly the same kind. 



'^ The truth is that an explanation of development is at present 

 beyond our reach. The controversy between preformation and 

 epigenesis has now arrived at a stage where it has little meaning 

 apart from the general problem of physical causality. What we > 

 know is that a specific kind of living substance, derived from the 

 parent, tends to run through a specific cycle of changes during which 

 it transforms itself into a body like that of which it formed a part ; : 

 and we are able to study with greater or less precision the mechanism 

 by which that transformation is effected and the conditions under 

 which it takes place. But despite all our theories we no more know 

 how the organization of the germ-cell involves the properties of the 

 adult body than we know how the properties of hydrogen and oxygen - 

 involve those of water. So long as the chemist and physicist are 

 unable to solve so simple a problem of physical causahty as this, 

 the embryologist may well be content to reserve his judgment on a 

 problem a hundred-fold more complex. — 



The second question, regarding the historical origin of the idio-"^ 

 plasm, brings us to the side of the evolutionists. The idioplasm of 

 every species has been derived, as we must believe, by the modifica- 

 tion of a preexisting idioplasm through variation, and the survival 

 of the fittest. Whether these variations first arise in the idioplasm 

 of the germ-cells, as Weismann maintains, or whether they may arise 

 in the body-cells and then be reflected back upon the idioplasm, is 

 a question to which the study of the cell has thus far given 

 no certain answer. Whatever position we take on this question, thei 

 same difficulty is encountered ; namely, the origin of that coordi- 

 nated fitness, that power of active adjustment between internal and 

 external relations, which, as so many eminent biological thinkers 

 have insisted, overshadows every manifestation of life. The nature 

 and origin of this power is the fundamental problem of biology.^ 

 When, after removing the lens of the eye in the larval salamander, 

 we see it restored in perfect and typical form by regeneration from 

 the posterior layer of the iris,'^ we behold an adaptive response to 

 changed conditions of which the organism can have had no antece- 

 dent experience either ontogenetic or phylogenetic, and one of so 

 marvellous a character that we are made to realize, as by a flash of 

 light, how far we still are from a solution of this problem. It may 

 be true, as Schwann himself urged, that the adaptive power _of 

 living beings differs in degree only, not in kind, from that of unor- 



1 Germinal Sdectiou, January, 1896, p. 284. 



2 See Wolff, '95, and Miiller, '96. 



2F 



