" Natural Selection " 57 



seen, there is a very definite limit, and this limit is 

 quickly reached." 



Now, what does this mean ? In the first place that 

 variation is a rare thing, and that when it does occur, it 

 is very soon extinguished, and secondly, that there is no 

 such thing as " natural selection," as all variations are 

 destroyed by the potent influence of marriage. 



Dewar and Finn conclude : " Isolation, then, is a 

 very important factor in the making of species, for 

 without it, in some form the multiplication of species 

 is impossible." Here we come to a crux, because if 

 true, then mutations must constantly occur and 

 " natural selection" must be the dominant law, which 

 accounts for the enormous number of species existing 

 on the earth. But where does isolation occur ? Is it 

 not another way of saying that environment does affect 

 species ? or, that species do adapt themselves to their 

 surroundings, such as climate, food supply, danger of 

 attack, and so on ? If so, that is a very different thing 

 from the suggestion in their conclusion, which means 

 that isolation prevails after mutations have occurred, 

 and these occur independently of environment and are 

 perpetuated in some mysterious way, when, as a 

 matter of fact, we know that all variations not induced 

 by environment are submerged by the influence of 

 marriage and the consequent return to the average. 



Dewar and Finn close their book with a very able 

 and interesting summary of the methods in which new 

 species are made. They say : " We have studied the 

 various factors of evolution — variation and co-relation, 

 heredity, natural selection, sexual selection, and 

 other kinds of isolation. How do these combine to 

 bring new species into being, and to establish the 

 same ? " In reply to this as regards natural selection 

 they say that "it is an important factor in evolution, 

 but not an indispensable one. Suppose there is no 



