101 THE AMERICAN NATURALIST [VOL. XL VII 



paper, I hope to show that the development of our knowl- 

 edge of correlation removes some other difficulties which 

 Darwin had to meet, and probably explains some other 

 facts which have been urged as supporting orthogenesis. 



Among the puzzles of evolution has been the steady 

 growth of rudimentary structures which have apparently 

 no function until they are considerably developed. I say 

 apparently no function, for the physiologist has learned 

 to be very cautious in saying that any part of the body 

 is without function or use. A few years ago it was quite 

 otherwise and it was supposed that various rudiments, 

 like the appendix, the hypophysis, the pineal gland, the 

 thymus and some other organs were without function; 

 the surgeons were busy explaining how much better we 

 were off without them ; and the anti-Darwinian was fond 

 of presenting these things as not consonant with the view 

 of adaptation. At the present time the uselessness of 

 these rudimentary structures is no longer affirmed. We 

 must therefore be very cautious in supposing that any 

 structure we see, no matter how insignificant it may 

 appear, is without importance. Darwin himself felt the 

 great fact of correlation, and his pangene theory was 

 invented, in part, to account for these facts. He would 

 be both astonished and delighted could he know how com- 

 pletely physiology has vindicated his appeal to correla- 

 tion as the explanation of some difficulties. 



Modern physiology has shown that the whole animal 

 organism is correlated by means of internal secretions; 

 that there is but one unit in the body, and that is the 

 whole organism. By the work of Knowlton and Starling 

 we now have the final proof of the correlation of the pan- 

 creas and muscles. The correlations between the hard 

 and soft parts of the body are of still greater importance 

 to the paleontologist, for it has been shown that the hard 

 parts are not independently variable, but that they are 

 dependent at every point upon the function of the soft, 

 internal organs. Who would have dreamed that the char- 

 acter of the skin, the hair, the shape of the skull, the in- 



