ORIGIN OF THE SPECIFIC TYPE 337 



long as they are contained in the germ-plasm in considerable numbers, 

 but they must become more and more infrequent in the germ-plasm 

 as successive generations are passed through the sieve of natural 

 selection, and the oftener these germ-plasms, to which the chances 

 of reducing division and amphimixis have assigned a majority of 

 the old determinants, are expelled from the ranks of the species 

 by personal selection. The oftener this has occurred in a species 

 the less frequently will it recur, and the more constant, ceteris paribus, 

 will the ' type ' of the species become. 



If we add to this idea the fact that adaptations take place very 

 slowly, and that every variation of the germ-plasm in an appropriate 

 direction has time to spread over countless hosts of individuals, we 

 gain some idea of the way in which new adaptations gradually bring 

 about the evolution of a more and more sharply-defined specific type. 



So far, however, we have only explained the morphological 

 aspect of the problem of the nature of species, but there is also 

 a physiological side, and for a long time this played an important 

 part in the definition of the conception of species. Until the time 

 of Darwin it was regarded as certain that species do not intermingle 

 in the natural state, and that, though they could be crossed in rare 

 cases, the progeny would be infertile. 



Although we now know that these statements are only relatively 

 correct, and that in particular there are many higher plants which 

 yield perfectly fertile hybrids, it is nevertheless a striking phenomenon 

 that among the higher animals, mammals, and birds the old law 

 holds good, and hybrids between two species are very rarely fertile. 

 The two products of crossing between the horse and the ass, the mule 

 and the hinny, are never fertile inter se, and very rarely with a member 

 of the parent stock. 



We have to ask, therefore, what is the reason of this mutual 

 sterility of species ; whether it is a necessary outcome of the morpho- 

 logical differences between the species, or only a chance accessory 

 phenomenon, or perhaps an absolutely necessary preliminary condition 

 to the establishment of species. 



The last was the view held by Eomanes. He believed that 

 a species could only divide into two when it was separated into 

 isolated groups either geographically or physiologically, that is, when 

 sexual segregation in some form is established within the species, 

 so that all the individuals can no longer pair with one another, but 

 groups arise which are mutually sterile. It is only subsequently, he 

 maintained, that these groups come to differ from one another in 

 structure. To this hypothetical process he gave the name of ' physio- 

 II. z 



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