THE GLANDS: THYROID AND PITUITARY 47 



a large skull, and a complicated development of brain, are the 

 diagnostic signs. Thyroid internal secretion has a very definite 

 controlling relation to all of them: to skin, its hairiness, mois- ~ 

 ture and amount of mucus, to the growth and size of the bones, 

 especially the bones of the extremities and the skull, and to 

 intelligence and the complexity of the convolutions of the brain.^. 

 Injury to the thyroid, especially in growing animals, is followed 

 by profound retrogression or arrest of development in skin,-,, 

 skeleton and brain. 



In the fishes and the cyclostomes the thyroid is represented 

 only by some small scrubby patches, little larger than the heads 

 of pins, scattered along the aorta, the great blood vessels from 

 the heart, and out a little way along each gill. It becomes larger 

 and more compact among the amphibians and reptiles, but still 

 remains quite small. Large and prominent among the birds and 

 mammalia, it is largest and most prominent among the primates 

 and man. It is hence permissible to think of the thyroid as a 

 dictator of evolution, to crown it as the vertebrate gland par 

 excellence, and to call the typical vertebrate brand marks sec- 

 ondary thyroid characteristics in precisely the sense of Darwin 

 classing the horns of cattle as secondary sexual characteristics. 



In such enthusiasm for the thyroid as a determinant of evolu- 

 tion, its pillar of cloud by day and column of fire by night, one 

 should not forget the other glands of internal secretion. In them 

 all, we may suppose, Life, tired of inventing merely prehensile, 

 destructive and reproductive organs, hit upon the happy thought 

 of contrivances which are in essence chemical factories to speed 

 up the rate of variation and so of a higher evolution. 



Creator of the Land Animal 



According to this conception the thyroid played a fundamental 

 part in the change of sea creatures into land animals. Experi- 

 mentally, thyroid has been used to transform one into the other. 

 Thus the occasional change of a Mexican axolotl, a purely aquatic 

 newt, breathing through gills, into the amblystoma, a terrestrial 

 salamander, with spotted skin, breathing by means of lungs, has 

 long been known. Feeding the axolotl on thyroid gland pro- 

 duces the metamorphosis very quickly, even if the axolotl is kept 

 in water. In the reptile house at the London Zoological Gardens 

 full-grown examples of the common black axolotl and the pretty 

 white variety are exhibited. Some are nearly three inches long. 



