350 



THE EVOLUTION THEORY 



spherical form, that is, composed of several ids whose ' plurivalence ' 

 cannot be directly recognized, but can only be inferred from their 

 further development; there are bivalent chromosomes of double 

 value and quadrivalent chromosomes of fourfold value, which we 

 have to think of as made up of two or four ids. It would lead 

 us too far to go into this more precisely, nor does it fall within 

 the scope and intention of these lectures to inquire into these intimate 

 and still disputed details. 



The germ-plasm of every species of plant or animal is thus 

 composed of a larger or smaller number of ids or primary constituents 

 of an individual, and it is through the co-operation of these that the 

 individual which develops from the ovum is determined. 



We have further to inquire what conception we can form of the 

 constitution of an id and of its mode of operation. I have already 

 spoken of ' primary constituents ' {Anlagen) of which the germ-plasm 

 consists, but what right have we to think of the parts of an animal 



Fig. 88. Sperm-mother-cells (spermatocytes) of the salamander. A, cross- 

 section of the cell in the aster-stage ; the chromosomes {chr) or idants do not 

 reveal that they are compounded out of many ids, which are, however, quite 

 distinctly seen in B (Jd), where the chromosomes or idants {chr) are already 

 longitudinally split, zk, cell- substance, csp, centrosome. c, centrosome in 

 division. After Hermann and Driiner. 



as already contained in the germ in any form whatever? Is it 

 not equally possible that the germ consists of parts, none of which 

 bear any definite relation in advance to the parts of the finished 

 animal? Might not the germ-cell, along with its nucleus, undergo 

 transformations and regular changes which would successively give 

 rise to new conditions, namely, the different stages of development, 

 until finally the complete animal was attained ? 



We stand here before an old problem, before the two opposed 

 interpretations — the theory of 'Evolution' and the theory of 'Epi- 

 genesis,' which were first ranged against each other long ago, and which 

 are a cause of strife even now, although in somewhat different guise. 



The theory of ' Evolution ' is especially associated with the name 

 of Bonnet, who elaborated it in detail in the eighteenth century. 



