SHARE OF THE PARENTS IN BUILDING UP THE OFFSPRING 45 



former kind are derived from two different ova, and that those of the 

 latter kind arise from a single ovum, which, after fertilization, has 

 divided into two ova. This not infrequently occurs in fishes and 

 other animals, and we can bring it about artificially in a number 

 of species by experimentally separating the two first blastomeres. 



We have here, then, a case of absolute identity of the germ-plasm 

 in two individuals, for the id-combination of the two ova derived 

 from the same process of fertilization must be exactly the same. 

 That in such a case, notwithstanding the inevitable differences of 

 external influences to which the twins are exposed from intra-uterine 

 life onwards, such a high degree of resemblance should arise is a fact 

 of great theoretical importance. From the basis of the germ-plasm 

 theory we can very well understand it, for, according to the theory, 

 only precisely similar combinations of ids can give rise to identical 

 individuals. 



But we learn more than this from the occurrence of identical twins. 

 They prove above all that the whole future individual is determined at 

 fertilization, or, to express it theoretically, that the id-composition of 

 the germ-plasm is decisive for the whole ontogeny. It might have 

 been supposed that the combination of ids could change again during 

 development, and that a greater multiplication of some than of others 

 might take place at certain stages of development, or through certain 

 chance external influences. It might have been thought that there was 

 a struggle among the ids in the sense that some of them were sup- 

 pressed and set aside. All such suppositions break down in face of 

 the fact of identical twins, which teaches us that identical germ-plasm 

 evokes an ontogeny which runs its course as regularly as two chrono- 

 meters, which are constructed and regulated alike. 



But when I say that a struggle of the ids, in the sense of a material 

 setting aside of some of them, cannot take place, I by no means intend 

 to maintain that the influence which each individual id exerts on the 

 course of development may not be disproportionate to that exerted 

 by others, and, under some circumstances, very disproportionate 

 indeed. I must refrain from entering into this subject in detail 

 now, but I should like to give at least an indication of what I 

 mean. 



If the germ-plasm consists of ids, these ids collectively must 

 determine the structure, the whole individuality — let us say, 

 briefly, the ' type ' of the offspring ; it is the resultant of all the 

 different impelling forces which are contained in the different ids. 

 If these were all equally strong, and all operating in the same 

 direction, they would necessarily all have the same share in the 



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