396 COOK 



appear quite uniformly intermediate in the first generation may- 

 prove, nevertheless, to be completely sterile, as in the mule, 

 whereas intermediate crosses between narrow varieties are always 

 completely fertile, more so, it may be, than their more inbred 

 parents. No distinction is to be drawn between crosses and 

 hybrids which are uniformly intermediate and at the same time 

 fertile, but there is a wide range of phenomena between an inter- 

 mediate, fertile cross between narrow varieties and an inter- 

 mediate sterile hybrid between diverse species. Next to the 

 hybrids which are intermediate, but sterile, are those which are 

 intermediate and fertile, but show diversity and partial sterility 

 in the second generation, proving that the parental elements did 

 not combine in a manner to afford a stable equilibrium of hered- 

 ity. In another stage of hybridity, with less diversity of parents, 

 the first generation is variable, which may be taken to mean that 

 the parental elements are sufficiently similar to influence each 

 other, instead of exerting a uniform degree of repulsion. 

 Nevertheless, they do not combine readily, but form uncertain 

 and extremely varied combinations. 



The purpose of this enumeration is to show that with hybrids, 

 as with crosses, there is a series of phenomena which can be 

 described and interpreted in terms of diversity, using as a stand- 

 ard the normal diversity of the individuals of species in nature. 

 In this way it is possible to avoid the ambiguities which have 

 attended the use of the false and artificial standard of uniformity. 

 From normal diversity there may be departures on either side, 

 on the one to abnormal uniformity, on the other to abnormal 

 diversity, and both of these can be reached, as we have seen, 

 in several ways. Uniformity appears : 



1. In closely selected varieties (stens). 



2. In varieties or individuals propagated from cuttings or by 

 other asexual methods (clones). 



3. In the progeny of inbred saltatory variations (mutations). 



4. In crosses between moderately inbred stenic varieties. 



5. In first generation hybrids between species so remote as to 

 combine with difficulty. 



Likewise diversity greater than the normal may appear : 

 1. Among mutations from narrowly inbred varieties. 



