18 The Problems 



know many cases where such a result is approached, and 

 very many where all the essential features which we regard 

 as constituting the characters of the breed are reproduced 

 with approximate certainty in every member of the pure- 

 bred race, which thus closely approach to uniformity. 



But if two germ-cells of dissimilar constitution unite 

 in fertilisation, what offspring are we to expect * ? First 

 let us premise that the answer to this question is known 

 experimentally to differ for many organisms and for many 

 classes of characters, and may almost certainly be in part 

 determined by external circumstances. But omitting the 

 last qualification, certain principles are now clearly detected, 

 though what principle will apply in any given case can only 

 be determined by direct experiment made with that case. 



This is the phenomenon of c ross-breeding. As generally 

 used, this term means the union of members of dissimilar 

 varieties, or species : though when dissimilar gametes t pro- 

 duced by two individuals of the same variety unite in 

 fertilisation, we have essentially cross-breeding in respect 

 of the character or characters in which those gametes differ. 

 We will suppose, as before, that these two gametes bearing 

 properties unlike in respect of a given character, are borne 

 by different individuals. 



In the simplest case, suppose a gamete from an in- 

 dividual presenting any character in intensity A unite in 

 fertilisation with another from an individual presenting 

 the same character in intensity a. For brevity's sake we 



* In all the cases discussed it is assumed that the gametes are 

 similar except in regard to the "heritage" they bear, and that no 

 original variation is taking place. The case of mosaics is also left 

 wholly out of account (see later). 



t The term "gamete" is now generally used as the equivalent of 

 "germ-cell," whether male or female, and the term "zygote" is here 

 used for brevity to denote the organism resulting from fertilisation. 



