3r)4 THE EVOLUTION THEORY 



It is, howcNcr, (^[uhv possible to leave aside for the present all 

 attempts at an explanation of life, and simply to take the elements 

 of life for <n-anted, and on tliis basis to build up a theory of heredit}'. 

 ^^^• have already taken a step in that direction by establishing that 

 tl„. wl u.lr substanc' nf the fertilized ovum does not take part in 

 heredi ty in the same de^rree. but that only a small part, the chroma tin 

 of the micleus, is to be loo ked upon as the bearer of the hered itary 

 iiualities, and by d educing, fu rther, that this chromatin is made u p of 



a ^vnn'"^"' nnmhpr nf small ]mh still visiV)1e units the ids, ennb of 



" 2 ~ — — 



whi ch virtually represents the whole organism, or, as I have a lready 

 expressed it, ea ch of which contains within it aalf ^ -u^ -^iiamary 

 consti tuents, all the parts of a perfect anim al. 



It was these ' primary constituents ' which led us to the 

 digression in regard to Bonnet's theory of ' Evolutio ' and Wolffs 

 ' Epigenesis.' 



Let us now inquire yvha.i must be. the con stitutiQn_nf ,such 

 a o jirnma.tin globule, an id. so that, shut up within the nucleus of 

 a living reproductive cell, it ofir\ direct the development of a new 

 organi sm which resembles its paren t. Two fundamental assumptions 

 present themselves, and these can be related to every conception of 

 a ' germ-plasm,' even independently of the assumption of ids. Either 

 w e may think of the id as made up of similar or of different kind s of 

 parts, noj2e_ _of which b f^^ »^y onrtnhmni vn1<itinTi tn ihf^ ptiTts of the 



p erfect animal , or we thin ^of it as composed of a mass of di fferent 

 kiii ds of parts, each of which hears a relation tn a 2)cirticular 'pa rt of 

 th e perfe ct animal, anj _so to some extent represents its ^prima ry 

 c onstituen ts ' (Anfagen), although there may be no resemblance 

 between these ' primary constituents ' and the finished parts. The 

 assumption of a germ-plasm composed of similar parts, which has 

 been made, for instance, by Herbert Spencer, may be called the 

 modern form of epigenesis, while the other assumption is the modern 

 form of the ' evolution ' theory. A s the forme rtheory can no longer 

 call to its aid a ' formative power' as aPeus jx ma.chima, it can only 

 e xplain d evelopment as induced by the influence of external 

 c onditions— temp .erature^ir, water, gravi ty, position of parts-^^u pon 

 the , chemical components of the germ-pla sm, which ar e everywhere 

 uniformly min.od ed ;. and it makes no difference whether this uniform 

 germ-plasm is thought of as composed of many different kinds of 

 parts, as long as those parts are mingled uniformly to make the germ- 

 plasm and bear no relation to definite parts of the developing animal. 

 Oscar Hei-twig has recently outlined such a theory. Although I cannot 

 expound it here I must say at least so much with regard to it, and to 



