2 THE EVOLUTION THEORY 



of this accessory idioplasm must, however, be looked for only in the 



egg-cell. 



In plants this bud-idioplasm must be complete germ-plasm, 



because the budding starts only from one kind of cell, the cambium- 

 cells ; but in animals in which— as it seems— it always proceeds from 

 at least two different kinds of cells— those of the ectoderm and those 

 of the endoderm— the matter is more complex. In this case these 

 two kinds of cells will contain as bud-idioplasm two different groups 

 of determinants, which mutually complete each other and form 

 perfect germ-plasm, and only the co-operation of these two sets will 

 give rise to the formation of a bud. I will not, however, go further 

 into detail in regard to these relations, for the theory can do nothing 

 more here than formulate what has been observed ; it is hardly in 

 a position to help us to a better understanding of the facts. 



The case is not much clearer in regard to the processes which 

 lead to the replacing of lost parts. The manifold phenomena of 

 regeneration can also be brought into harmony with the theory, if we 

 attribute to those cells from which the replacing or entire reconstruc- 

 tion of the lost part arises an ' accessory-idioplasm,' which, at least, 

 contains the determinants indispensable to the building up of the part. 

 It is possible that the assumed accessory idioplasm frequently contains 

 a much larger complex of determinants, and that it depends on the 

 liberating stimuli which, and how many of these, will become active. 



If we take a survey of regenerative phenomena in the animal 

 kingdom, it strikes us at once that the capacity is very different in 

 different species, extraordinarily great in some and very slight in 

 others. In general it is greater in lower animals than in higher, but, 

 nevertheless, the degree of differentiation cannot be the onl}" factor 

 that determines the capacity for regeneration. Tliat unicellular 

 organisms can completely replace lost parts, that even a piece of an 

 infusorian can reconstruct the whole animal if only the piece contain 

 a part of the nucleus, we have already seen when discussing the signi- 

 ficance of the nuclear substance. In this case the nucleus must 

 contain the complete germ-plasm, that is, the collective determinants 

 of the species, and these induce the reconstruction of the lost part, 

 though they do so in a way that is still entirely obscure to us. In 

 the meantime, our interpretation will not carry us further, either here 

 or in regard to any other order of vital phenomena. To go further 

 would be little short of propounding a causal theory of life itself ; it 

 would mean having a complete and real 'explanation' of what 'life' is. 

 As yet no one has been able to claim this position. We can see the 

 different stages through which every organism passes, and that they arise 



