BLASTOOENIC VARIATIONS. 115 



supply of fresh combinations of individual variations, as 

 Weismann maintains? Exact numerical evidence upon 

 the point is, indeed, almost entirely wanting, but one's 

 own everyday experience is all in favour of its validity. 

 Thus one knows that animals of the same litter, which 

 during embryonic development must have been exposed 

 to very nearly equal environmental conditions, differ al- 

 most as much from each other as from animals of for- 

 mer litters, and in many cases not very much less 

 than from animals in the litters of entirely different 

 parents. 



ISTow, as this phenomenon is one of almost universal 

 occurrence, it cannot be maintained that the observed 

 variations may be brought about by chance differences 

 of environmental conditions acting during development. 

 They must obviously be in chief part the result of dif- 

 ferences in the individual sex-cells from which the off- 

 spring took their rise. So important is this conclusion 

 that it was enunciated by Victor Hensen as a funda- 

 mental law of amphigonic heredity. This has been 

 thus worded by Weismann:* " The individual is deter- 

 mined at the time of fertilisation, or, in other words, 

 the individuality of an organism results from the fact 

 that the germ-plasm is composed of the paternal and 

 maternal ids which are brought together in the egg- 

 cell." 



Very interesting evidence in favour of this law is 

 furnished by cases of identical human twins. It has 

 long been known that whilst the larger number of 

 twins show no greater resemblance to each other than 

 do children of the same parents born consecutively, a 

 * " Germ-plasm," p. 253. 



