CLASSIFICATION AND EVOLUTION 527 



food, enemies, etc, will allow. A pair of robins will 

 produce ten or more young in a year, yet, since the number 

 of robins does not increase, only two of these can survive. 

 This is an exceptionally small death-rate. Many animals 

 produce thousands of offspring : the blow-fly, for instance, 

 gives rise to a progeny of 20,000. Now it is impossible 

 to believe that the destruction which this involves is 

 altogether haphazard. Some of the individuals will be 

 feebler, or slower, or less cunning, or less protectively 

 coloured, or less warmly clad than the rest, and it is 

 certain that these will as a rule be the first to be destroyed, 

 and that the survivors will generally be above the average 

 of the rest in regard to the characters in which selection 

 has taken place. (3) The alteration which is thus made 

 in the average characters of the species will be maintained 

 by the action of heredity. It is often alleged as an 

 objection to the theory of evolution by natural selection 

 that any large variation in a favourable direction, though 

 it may lead to the survival of the individual in which it 

 occurs, will nearly always be weakened in the next 

 generation by what is known as the " swamping effect of 

 intercrossing." That is to say, the exceptional individual 

 will probably mate with an average member of the species 

 and their offspring will be intermediate between them, 

 and so. in a few generations the favourable variation will 

 have become so slight as to give no effective advantage 

 in the struggle for existence. This difficulty, however, 

 disappears in the light of discoveries which have been 

 made by the students of heredity known as the 

 Mendelian school, after the discoverer of the principles 

 upon which they rely. We have already seen (p. 197) that, 

 in many cases, when two individuals which differ in any 

 respect breed together, their features do not blend, but 

 the peculiarity of one parent is " dominant " in the 

 offspring, while that of the other is latent or " recessive." 

 For instance, the offspring of a wild grey rabbit and the black 

 variety are all grey. Thus the dominant variety remains 

 distinct and is not weakened by crossing. If, however, 

 the offspring of such a cross be bred together, it will be 

 found that the recessive variety has also not been ex- 

 tinguished, for on the average a quarter of the generation 



