306 THE ZOOLOGIST. 



may confer height, colour, shape, instincts, powers both of mind 

 and body ; indeed, so many of the attributes which animals and 

 plants possess that we feel justified in the expectation that with 

 continued analysis they will be proved to be responsible for most if 

 not all of the differences by which the varying individuals of any 

 species are distinguished from each other. I will not assert that 

 the greater differences which characterize distinct species are due 

 generally to such independent factors, but that is the conclusion to 

 which the available evidence points. All this is now so well under- 

 stood, and has been so often demonstrated and expounded, that 

 details of evidence are now superfluous. 



But for the benefit of those who are unfamiliar with such work 

 let me briefly epitomise its main features and consequences. Since 

 genetic factors are definite things, either present in or absent from 

 any germ-cell, the individual may be either "pure-bred" for any 

 particular factor, or its absence, if he is constituted by the union of 

 two germ-cells both possessing or both destitute of that factor. If 

 the individual is thus pure, all his germ-cells will in that respect 

 be identical, for they are simply bits of the similar germ-cells which 

 united in fertilization to produce the parent organism. We thus reach 

 the essential principle, that an organism cannot pass on to offspring 

 a factor which it did not itself receive in fertilization. Parents, 

 therefore, which are both destitute of a given factor can only pro- 

 duce offspring equally destitute of it ; and, on the contrary, parents 

 both pure-bred for the presence of a factor produce offspring equally 

 pure-bred for its presence. Whereas the germ-cells of the pure- 

 bred are all alike, those of the cross-bred, which results from the 

 union of dissimilar germ-cells, are mixed in character. Each posi- 

 tive factor segregates from its negative opposite, so that some germ- 

 cells carry the factor and some do not. Once the factors have been 

 identified by their effects, the average composition of the several 

 kinds of families formed from the various matings can be predicted. 



Only those who have themselves witnessed the fixed operations 

 of these simple rules can feel their full significance. We come 

 to look behind the simulacrum of the individual body, and we 

 endeavour to disintegrate its features into the genetic elements by 

 whose union the body was formed. Set out in cold general 

 phrases such discoveries may seem remote from ordinary life. 

 Become familiar with them and you will find your outlook on the 

 world has changed. Watch the effects of segregation among the 

 living things with which you have to do — Plants, Fowls, Dogs, 

 Horses, that mixed concourse of humanity we call the English 

 race, your friends' children, your own children, yourself — and how- 

 ever firmly imagination be restrained to the bounds of the known 

 and the proved, you will feel something of that range of insight 

 into nature which Mendelism has begun to give. The question is 

 often asked whether there are not also in operation systems of 

 descent quite other than those contemplated by the Mendelian rules. 

 I myself have expected such discoveries, but hitherto none have 

 been plainly demonstrated. It is true we are often puzzled by the 



