The reason why it is so easy to attack your theory is that it is so easy to confuse the survival of an _individual_ with the survival of a _peculiarity_ of _type_. No one has ever said that an _individual_ is _assisted_ by the possession of selective fertility: that is a matter which cannot affect his chance of _life_. Nor has any one said that the possession of selective fertility in an _individual_ will _of itself_ increase the chance of his having _progeny_ that will survive, and in turn become the progenitors of others that will survive. Taken by itself, the fact that an _individual_ is capable of fertility with some only of the opposite sex lessens the chance of his having progeny. Whether or not he is more or less favourably situated than his _confreres_ for the battle of life must be decided by the _total sum_ of his peculiarities; and the question whether or not this selective fertility will be a hindrance must be decided by considerations depending on the other peculiarities associated with it. But when we come to consider the survival or permanence of a _type_ or _peculiarity_, the case is quite different. It then becomes not only a favourable circumstance, but, in my opinion, almost a necessary condition, that the peculiarity should be associated with selective fertility[64]. [64] As, for example, in the case of sexuality in general. It is not to the advantage of such individual male Arthropoda as perish after the performance of the sexual act that they should perform it; but its performance is necessary for the perpetuation of their species.--G. J. R. Take the case of the Jews. I don't think that intermarriage with other nations would lessen their fertility, or diminish the number of their progeny; nor is there any reason to think that this progeny would be unequal to the struggle for existence. But no one doubts that the abandonment of their voluntary isolation (which operates so far as this is concerned as a selective fertility), would lead to the disappearance of the familiar Jewish type. All the world would get some of it; but as a whole it would be "swamped." Now although no doubt Wallace would admit all this, he fails to give it the weight it ought to have. In discussing the question of its operation he considers too exclusively the case of the individual.