CHAPTER II 



It has already been pointed out, that the conditions 

 of life are usually such as to cause the number of traits 

 necessary for survival to be numerous, and that this 

 again causes the process of evolution to be slow, though 

 it is no bar to it as some biologists have supposed. 

 But hovsrever slow the multiplicity of essential traits 

 may render evolution, it is difficult at first sight to 

 understand why evolution should be so extremely slow 

 as it generally is. Every plant or animal pair usually 

 gives origin to numerous offspring, sometimes the off- 

 spring are numbered by millions (e. g. the offspring of 

 cod-fish) ; yet the nfimber of individuals in each species 

 does not increase as a rule, whence it is clear that 

 nature, by destroying the majority of the offspring, 

 exercises her power of selection with extreme stringency. 

 Under the circumstances we might d, p^'iori expect 

 evolution to be rapid. But what are the facts ? Some 

 types, such as the lamp-shells, have persisted almost 

 unchanged during enormous epochs of time, and in 

 almost all cases nature requires hundreds, nay thousands 

 or tens of thousands of years to bring about compara- 

 tively trifling changes of structure. Why is this ? 



The answer is probably found if we consider certain 



groups of apparently unconnected facts. Children who 



do. not especially resemble their parents often resemble 



their grandparents, or even more remote ancestors ; at 



any rate they more often resemble an ancestor than 



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