254 



THE GERM-PLASM 



external influences of nutrition. &c.. to which the germ is sub- 

 ject after fertilisation. The existence of 'identical' human 

 twins, however, proves the contrary. Some twins do not re- 

 semble one another more closely than do children of the same 

 parents which are born consecutively ; and. in fact, this is 

 apparentlv tnie of the greater number of twins, in which the 

 dissimilarity may even be very considerable. We have every 

 reason to suppose that such • dissimilar ' twins are usually derived 

 from tivo ova, which must of course have been fertilised by two 

 different spermatozoa. On the other hand, in the case of those 

 twins which I speak of as • identical." the resemblance, although 

 not perfect, is much closer than has ever been observed in 

 children born successively. There is every reason to suppose 

 that such identical twins are derived from a single ovum and 

 spermatozoon. If this is actually the case, it furnishes a proof 

 of the above statement that heredity is potentially decided at the 

 time of fertilisation, or, expressed in terms of the idioplasm, that 

 the nature of the combination of the parental ids which takes 

 place during fertilisation predetermines the whole subsequent 

 ontogeny. The slight differences which exist between identical 

 twins would therefore probably indicate to what extent the course 

 of development may be affected by external influences. These 

 differences are generallv so slis;ht that it is difificult to observe 

 them at all, unless they are specially sought for ; as a njle such 

 twins can only be individually recognised by the parents or 

 brothers and sisters, and cannot be distinguished from one 

 another by strangers. 



These slight differences might, however, be due to an imper- 

 fect predetermination of the influence which is exerted at every 

 ontogenetic stage by the idioplasm of each of the parents. We 

 can hardly decide between these views from the consideration of 

 identical twins only. Mr. Otto Ammon, of Karlsruhe, has kindly 

 furnished me with two photographs of identical twins taken in 

 consecutive years at the ages of seventeen and eighteen, as well 

 as with exact measurements of all parts of their bodies. In 

 spite of a striking resemblance, not only in face but in all parts 

 of their bodies, certain differences are nevertheless recognisable 

 between them. For instance, the height of the one marked 

 No. 507 on Mr. Ammon\s list, measured, when lying down, 172 

 cm., and that of No. 508 only 170 cm.: and again, although 

 the length of the hand and of the left arm is the same in both, — 



