566 THE EVOLUTION OF LIFE. 



life has existed — one hundred million years. Suppose we 

 divide that time into as many parts as there are hours occu- 

 pied in the development of a human foetus. And suppose 

 that during these hundred million years there has been going 

 on with some uniformity the evolution of the various organic 

 types now existing. Then the amount of change undergone 

 by the foetus in an hour, will be equivalent to the amount 

 of change undergone by an evolving organic form in fifteen 

 thousand years. That is to say, during general evolution it 

 may have taken fifteen thousand years to establish, as dis- 

 tinct, two species differing from one another no more than 

 the foetus differs from itself after the lapse of an hour. 

 Hence, though we lack proof that adaptive modifications be- 

 come specific traits, it is quite possible that they are in course 

 of becoming specific traits. 



The converse proposition, that the traits by which species 

 are ordinarily distinguished are non-adaptive traits is well 

 sustained; and the statement that, if not themselves useful 

 they are correlated with those which are useful, is, to say the 

 least, unproved. For the instances given by Mr. Darwin of 

 correlated traits are not those between adaptive traits and 

 the traits regarded as specific, but between traits none of 

 which are specific; as between skull and limbs in swine, 

 tusks and bristles in swine, horns and wool in sheep, beak 

 and feet in pigeons. 



If we seek a clue in those processes by which correlations 

 are brought about — the physiological actions and reactions — 

 we may at once see that any organic modification, be it 

 adaptive or not, must entail secondary modifications through- 

 out the rest of the organism, most of them insensible but 

 some of them sensible. The competition for blood among 

 organs, referred to above, necessitates that, other things re- 

 maining the same, the extra growth of any one tells on 

 all others, in variable degrees according to conditions, and 

 may cause appreciable diminutions of some. This is not 

 all. While the quantity of blood supplied to other organs is 



