No. 554] ADAPTATION AND THE PHYSIOLOGIST 102 



telligence itself, the length of the limbs, or the speed of 

 transformation of a tadpole into a frog would be depend- 

 ent on the thyroid or thymus gland? That the minute 

 parathyroids should be absolutely necessary to the life of 

 an organism thousands of times their weight? Or that 

 the development of the testes, the change of milk teeth 

 to the permanent dentition, the growth of the bones of 

 the extremities, should be dependent on the anterior lobe 

 of that apparently useless rudiment, the hypophysis? Or 

 that the secretion of milk and urine should depend on the 

 posterior lobe of the same organ? Who had the temerity 

 to suggest that the corpus luteum should be influencing 

 the development of the mammary glands ? Do we not see, 

 indeed, that most of the characters of the body which 

 have steadily developed from the fishes to man are 

 secondary characters dependent on the anterior develop- 

 ment of these ductless glands? Is this fact without sig- 

 nificance to the paleontologist in helping him to under- 

 stand the apparent steady progress in one direction, the 

 appearance of orthogenesis? It will be asked, perhaps, 

 what has caused the steady development of these glands. 

 But the answer is not difficult. They are, in their turn, 

 parts of the mechanism of adaptability which has been 

 consistently selected in evolution. They are concerned 

 not only in the growth of bone, but in the growth of the 

 nervous system, the heat control of the body, the im- 

 munity mechanisms, the efficiency of muscles, and are in 

 the chain of reproduction itself. These facts largely 

 remove, in my opinion, the difficulties in understanding 

 how rudimentary organs could be useful. 



But not only do these facts remove these difficulties in 

 the way of the selection theory, but they have a no less 

 important bearing on the problem of heredity. They 

 show that there can be no independently variable quali- 

 ties in the animal body. The body is a unit, and I, at 

 least, can imagine no part of it which can vary without 

 influencing other parts. Correlations are everywhere. 

 Pigment is often cited as a unit character, but how can it 



