PREFORMATION AND EPIGENESIS 329 



epigencsis has now arrived at a stage where it has Httle meaning 

 apart from the general problem of physical causality. What we 

 know is that a specific kind of livdng 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 properties of the idioplasm involve 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 causality as this, 

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

 problem a hundredfold 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 pre-existing 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 on which, as far as I can see, the study of the cell has 

 not thus far thrown a ray of light. Whatever position we take on 

 this question, the same difficulty is encountered; namely, the origin 

 of that co-ordinated 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 w^e 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- 

 ganized bodies. It is true that we may trace in organic nature long 

 and finely graduated series leading upward from the lower to the 

 higher forms, and we must believe that the wonderful adaptive mani- 

 festations of the more complex forms have been derived from simpler 

 conditions through the progressive operation of natural causes. But 



1 See Wolff, '95, and MuUer, '96. 



2 See Wolff, '94, for an admirably clear and forcible discussion of this case. 



