22 VERTEBRATE ZOOLOGY 



ation. The more brilliant pigments may be looked upon as end prod- 

 ucts of certain chemical processes in metabolism, and therefore 

 appropriate in groups that represent end products of long lines of 

 specialization. 



Possible Internal Causes of Some Phases of Senescence.— 

 The effects of atrophy or hypertrophy of such endocrine glands as 

 the thyroid, pituitary body (hypophysis), ovaries or testes, upon 

 growth and differentiation, are well known. Disturbances in the nor- 

 mal functionality of these various glands underlie giantism, dwarfism, 

 degeneracy, failure to complete the differentiation of the secondary 

 sexual characters, etc. It is not unUkely that racial senescence, like 

 individual senescence, may be the result of a progressive deteriora- 

 tion of the germinal determiners of these important balance-wheels of 

 organic growth. It is also not unlikely that most of the changes in 

 bodily proportion, which constitute a vast preponderance of specific 

 and racial differences, are largely due to relative racial atrophy or 

 hypertrophy of these growth-regulating glands. The giant races may 

 be those in which thyroid abnormality has unloosed the restraints to 

 growth, and size has reached the limit of mechanical possibility. Sim- 

 ilarly racial thyroid or hypophysis atrophy may account for degen- 

 erate types. An excellent example of the effectiveness of thyroid in 

 controlling differentiation is seen in the case of frog development. 

 The bull-frog larva in northern waters takes several years to reach the 

 stage of metamorphosis; but if fed upon thyroid there is a precocious 

 metamorphosis of the first-year larva while it is still only about one- 

 fourth the size normally reached in nature. Thus giantism in frog 

 larvae is the result, presumably, of deficient or belated thyroid secre- 

 tion. The converse of this experiment has been made by extirpation 

 of the hypophysis in tadpoles. Allen has found that these operated 

 larvse do not develop a thyroid and are unable to metamorphose. 

 The bearing of these experiments on the phenomenon of neoteny or 

 pedogenesis is obvious. 



VERTEBRATE PHYLOGENY 



Phylogeny may be defined as the science of ancestries or genealogies, 

 and, as such, is one of the chief concerns of the present book. In 

 attempting to discover the origin, ancestry, and relationships of a 

 given group of animals, we employ mainly three sets of criteria, which 

 for the sake of brevity we may term: homologies, ontogenies, and 



