400 THE EVOLUTION THEORY 



wore not an indispensaWo assumption in the explanation ol' uiotn- 

 niorphosis anil other plienoniena of development, I should rei;ai'd an 

 attempt at a theory ol" development without determinants as justiK- 

 nble. But I am forced to see in this fact alone an invalidation of all 

 eiaigenetic theories of doveloiDment, that is, of all theories ^vhit']l 

 assume a germ-siilistaiice without primary constituents, wliich can 

 produce the complicated body solely by varying step by stc^p under 

 the influence of external influences, both extra- and intra-somatic, 

 It is possible to conceive of an ovum in which tlio living substance 

 is of such a kind that it must vary in a definite manner under tlie 

 influence of warmth, air, pressure, and so on, that it must divide into 

 similar, and subsequently also into dissimilar parts, which then inter- 

 act upon each otlior in diverse ways and give rise to further varia- 

 tions, which in their turn result in differentiations and variations, till 

 ultimately we have the whole complicated organic machine complete 

 and ' finished ' in every detail. Certainly no mortal could make any 

 pronouncement as to the constitution of such a substance, but C'veu if 

 wo assume it, for the nonce, as possibl(\ how can we account for the 

 transmissible variation of the individual parts and developmental 

 stages, on which the whole phylogenetic evolution depends ? 



As the development of the butterfly exhibits the three main 

 stages of caterpillar, pupa, and perfect insect, each of which is inde- 

 pendently and hereditarily variable, and tliorel'oi'e implies a some- 

 thing in the germ, whose variation brings about a change in the one 

 stage only, so the ontogeny ol' every higher animal is made up of 

 numerous stages, which are all capable of independent and trans- 

 missible variation. How else should we human beings, in our 

 eiiibiyonic phase, still possess the gill-arches of our fish-like ancestors, 

 although much modified and without the gills ?• Truly, he who would 

 seek to deny that the stages of individual development are capable of 

 independent and transmissible variation must know very little about 

 embryology. Hut if the facts are as stated, how can they be recon- 

 ciled with the concei^tion ol' a germinal substance developing in epi- 

 genetic fashion? Every variation in this substance would affect not 

 only the whole mvceadoib of stagen, hut the whole orgunmii, with all 

 itsjKiHs. In this way too, then, we are driven to the conclusion that 

 there must be something in the geim whose variation causes variation 

 only in a particular part of a particular stage. This something we 

 define in our conception of the ' primary constituents ' (Anlagen)— the 

 determinants. These are not to be thought of either as 'miniatui'e 

 models,' or even as the 'seeds' of the parts; they alone cannot pro- 

 duce the part which they determine, but they oft'ect changes in the 



