400 THE EVOLUTION TIIEOKY 



-were not an iiulispensaltle assumption in the explanation of meta- 

 morphosis anil other phenomena of development, I should regard an 

 attempt at a theory of development without determinants as justifi- 

 ahli'. Hut T am forced to see in this fact alone an invalidation of all 

 epi^enetic theories of development, that is, of all theories which 

 assume a germ-substance without primar}?^ constituents, which can 

 proiluce the complicated body solely by varj^ing step by step under 

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

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

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

 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 other 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 an}^ 

 pronouncement as to the constitution of such a substance, but even if 

 we assume it, for the nonce, as possible, how can we account for the 

 transmissible variation of the individual parts and developmental 

 stages, on which the whole phylogenetic evolution depends ? 

 p As the development of the butterfly exhibits the three main 

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

 pendently and hereditarily variable, and therefore implies a some- 

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

 stage only, so the ontogeny of 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 

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

 although much modified and without the gills 1 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. But if the facts are as stated, how can they be recon- 

 ciled with the conception of a germinal substance developing in epi- 

 genetic fashion ? Every variation in this substance would affect not 

 only the whole succession of stages, but the tvliole organism ivith all 

 its 2^arts. In this way too, then,(^^e are driven to the conclusion that] 

 there must be something in the germ whose variation causes variation / 

 only in a particular part of a particular stage!^ This something wej 

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

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

 models,' or even as the * seeds ' of the parts ; th ey ^pn p ^ annot pro - 

 du ce the part which they de termine, but they effect changes iii_Jl 



