602 THE PHYSIOLOGY OF REPRODUCTION 



fertility/ but that cross fertilisation between different species 

 is frequently difficult to accomplish while there is every gradua- 

 tion between a mere disincUnation towards gametic union and 

 complete cross sterihty. 



The differences in fertihty between varieties and species 

 when crossed are discussed at some length by Darwin,^ who 

 summarises his general conclusions under seven heads. Firstly, 

 the laws governing hybridisation in plants and animals are 

 practically identical. Secondly, there are all degrees of cross 

 infertihty. " Thirdly, the degree of sterihty of a first cross 

 between two species does not always run strictly parallel with 

 that of their hybrid offspring. Many cases are known of species 

 which can be crossed with ease, but yield hybrids excessively 

 sterile ; and conversely some which can be crossed with great 

 difficulty, but produce fairly fertile hybrids. This is an inex- 

 phcable fact on the view that species have been specially 

 endowed with mutual sterihty in order to keep them distinct." 

 Fourthly, the degree of sterihty is often different in the two 

 sorts of reciprocal crosses between the same species, and hybrids 

 produced from reciprocal crosses sometimes differ in their degree 

 of sterihty. " Fifthly, the degree of sterihty of first crosses 

 and of hybrids runs, to a certa,in extent, parallel with the general 

 or systematic afiinity of the forms which are united. For 

 species belonging to distinct genera can rarely, and those 

 belonging to distinct famihes can never, be crossed. The 

 parallehsm is, however, far from complete ; for a multitude 

 of closely alhed species will not unite, or unite with extreme 

 difficulty, whilst other species, widely different from one another, 

 can be crossed with perfect facihty. Nor does the difficulty 

 depend on ordinary constitutional differences, for annual and 

 perennial plants, deciduous and evergreen trees, plants flowering 

 at different seasons, inhabiting different stations, and naturally 

 hving under the most opposite chmates, can often be crossed 

 with ease. The difficulty or facihty depends exclusively on the 



' Frazer has shown that this fact is probably the biological basis for the 

 practice of exogamic roarriages originally adopted by primitive races of 

 mankind and perpetuated under the iniluenoe of natural selection ( Totemiam 

 and Exogamy, London, 1910). 



^ Darwin, loc. cit. See also Origin of Species, 6th Edition, London, 

 1872. 



