ISOLATION 157 



3. Development proceeds further, and a feeble or mal- 



formed F x may be produced. 



4. Well-developed, more or less vigorous hybrids are 



produced which are sexually abnormal — e.g. one sex 

 missing from brood, spermato- and ovo-genesis 

 abnormal, production of intersexes, etc. 



5. F x more or less fertile — e.g. fertile with one sex of one 



of the parent species. 



6. F x fertile, but F 2 infertile or weakly. 



7. Complete fertility. 



In noting this wide range of possibilities, it is important 

 to remember that some degree of intraspecific sterility is 

 always met with. Sometimes sterility between certain types of 

 individuals is very marked — e.g. in some Ascidians (cf. Plough, 

 1930, 1932). The nearly fertile interspecific hybrids, there- 

 fore, grade over completely into species in which intraspecific 

 sterility is normally present in some degree. 



The essential question is whether any of these forms of 

 sterility provides commonly the first important stage in isola- 

 tion. At least one case is known of extreme sterility between 

 a species and a mutant differing only in a single character, 

 viz. Drosophila obscura (Lancefield, 1929), in which a naturally 

 occurring race, with a very large Y-chromosome, will not cross 

 with the normal form. It is difficult to see how a mutant 

 determining sterility could establish itself in the population ; 

 the process is not likely to be very common. On the whole, 

 however, the facts do not suggest that sterility is commonly 

 the initial method by which isolation is established ; at any rate 

 it is unlikely to have been the only important factor. This 

 subject has been discussed at greater length by Robson (1928). 



Conclusions 



This survey of the factors which promote isolation suggests 

 the following generalisations : 



(1) There is no one predominantly important way in 



which isolation becomes established in the early 

 stages of species-formation. 



(2) Geographical or temporary isolation is undoubtedly 



very important, but it cannot be claimed that this 



