SHARE OF THE PARENTS IN BUILDING UP THE OFFSPRING 49 



or ' type ' of the descendant is determined at fertilization, we may 

 be inclined to regard such an oscillation of the hereditary tendencies 

 as almost impossible, for it means that, with the given mingling of 

 parental germ-plasms, the potency of inheritance from the two parents 

 in every part of the offspring is determined once for all in advance. 

 But the case of identical twins corroborates these oscillations, for 

 in them, too, the father predominates in one part, the mother in 

 another, and it proves, at the same time, that these oscillations 

 do not depend on any chances whatever in development, but that 

 they are exactly predetermined in the mingling of the hereditary 

 substances in the germ-plasm of the fertilized ovum, and are strictly 

 adhered to throughout development. 



This fact can only be explained thus : the primary constituents 

 of the different parts and characters of the body are contained in 

 the parental germ-plasm in varying degrees of hereditary or trans- 

 missive sti'ength, and this can be understood very well from our 

 point of view without putting anything new into the ' portmanteau ' 

 of our theory (Delage). 



But I must digress a little in order to make this plain. 



When, in speaking of plant -hybrids, I said that the collective 

 ids of the germ-plasm of a species must be equivalent in regard 

 to the characters of the species, I did not speak quite precisely ; 

 in the majority of ids, in many cases in an overwhelming majority, 

 this must be the case, but not actually in all, at least not on the 

 assumption we make that the transformation of species is accomplished 

 under the control of natural selection. 



Let us recall what we have already established in regard to the 

 evolutionary power of natural selection, namely, that the changes 

 which it controls can never transcend the range of their utility, 

 and it will be clear to us that, of the many ids which make up the 

 germ-plasm of the species, only so many will be modified as are 

 necessary to evoke the character which has varied. Just as the 

 protective resemblance of an insect to a leaf may be raised to a very 

 high pitch, but can never become perfect, because an imperfect 

 resemblance is already sufiicient to deceive the persecutor, and the 

 selective process comes to a standstill because individuals which 

 possessed a still greater resemblance to a leaf would be no better 

 protected from destruction than the others, so in the modification 

 of a species the whole of the ids need not at once be modified, if 

 a majority is sufficient to stamp the great majority of individuals 

 with the desired variation. But it may happen that, at the reduction 

 of ids during the development of the germ-cells, an id-combination 

 II. E 



