214 



THE AMERICAN NATURALIST [Vol. XLIII 



describes this principle as an unknown law of growth. 

 It is to this aspect of the subject that I wish to direct 

 your attention. 



Only a superficial acquaintance with organisms is 

 needed to make one familiar with many examples of cor- 

 relation. A hairy integument is always associated with 

 mammary glands; albinism in fur and skin is accom- 

 panied with a red color in the eye; and many other ex- 

 amples of correlated characters might be given. The 

 question that we have to consider is the nature of the 

 association in correlated characters and much light can 

 be thrown upon this, I believe, by a study of the ductless 

 glands. 



The ductless glands such as the thyroid, the suprarenal 

 bodies and the hypophysis of the brain were originally 

 supposed to be functionless, but recent work has shown 

 them not only to be functional but absolutely essential 

 to the continuance of life. The removal of the supra- 

 renal bodies from a mammal is invariably followed by 

 death within a few hours and the loss of the hypophysis 

 or the thyroid is also fatal though only after a somewhat 

 longer interval. It is thus quite evident that these or- 

 gans are of vital importance and that the continuance 

 of life is dependent upon their presence. But they are 

 not only necessary for life ; they profoundly influence the 

 form and structure of the organism in which they occur. 

 This is best seen in the case of the thyroid. In extreme 

 disease of this gland or after its removal in the higher 

 mammals, the skin thickens and thus produces a mis- 

 shapen aspect in the features and the extremities, there 

 is a tendency to the loss of hair, and the nervous organs 

 are so affected that the animal sinks into a condition of 

 semi-idiocy (cretinism). Thus there are not only funda- 

 mental internal changes, but the external features such as 

 a naturalist might use in describing a species are pro- 

 foundly modified. Certain external features, then, in 

 the normal animal are correlated with the state of the 

 thyroid and, as disease and experiment show, they fluctu- 



