152 Evolution of Animal Life. 



which determine, among the " fittest survivors/' which shall 

 breed true, and so transmit their fitness. This hypothesis 

 is based on a fact which may be added to our accepted cata- 

 log-ue (though it is included under K), namely, the frequent 

 intersterility of individuals of the same species, and the cor- 

 responding interfertility of the same individuals otherwise 

 grouped. The causes are both physiological and, among 

 the higher animals, psychological. The former are almost 

 wholly unknown to us ; the latter are evinced in repugnance 

 and avoidance between the individuals. Whatever the 

 causes, the phenomenon is undeniable. 



Now the reproductive system of animals is most sensitive 

 of all to causes of change ; and it is not improbable that in 

 every generation of a wild species there exists this partial 

 intersterility. Let us suppose, then, that out of a million indi- 

 viduals competing for life in an environment which will sup- 

 port 400,000, say 100,000 of the survivors possess a small 

 advantageous peculiarity, while the other 300,000, though 

 at some disadvantage, manage to live to the next generation. 

 That is, there are 100,000 " fit " survivors, and 300,000 lucky 

 ones. Assume that, out of this 100,000, there happen to be 

 1000 individuals, who can or will pair with one another 

 only. The rest breed freely with the unprotected but for- 

 tunate 300,000, and the next generation gives us 1,000,000 

 individuals again, of which say 2500 are the offspring of 

 the close-breeding 1000. Perhaps only half of these retain 

 both the protective peculiarity and the protective sterility 

 or aversion. But it Avill easily be seen that while natural 

 cross-breeding obliterates in each generation the majority 

 of the variations, there is a protected close-breeding going 

 on, Avhich, if it only produces, at last, a single pair with 

 well-marked and permanent peculiarities, and sterile toward 

 the rest, has given the condition for a new species. 



And this process shortens immensely the time required. 

 We know by experience how quickly a new species of su- 

 perior fitness will exterminate or drive out all others. 



Migration is thus not necessary as an element of pre- 

 servation to the fittest. It is the inferior which must run 

 aAvay. 



Again, this theory accounts for the preservation of pro- 

 tective specific characters. It permits even the formation 

 of new specific characters not protective. 



