16 FACTORS IX VARIATIOX. 



gland ciiid the suprarenal bodies), only investigated in 

 recent years, show how greatly an organism may be 

 modified from within. Prof. Arthur Keith, in his address 

 to the Anthropological section of the British Association 

 in 1919,* reviewed the morphogenetic mechanism of these 

 glands. He gives the opinion that a reduction or alteration 

 in the activity of the thyroid has been a factor in deter- 

 mining some of the characteristics of the Mongol and Negro 

 races and supports the theor}' that " the conformation of 

 man and ape and of every vertebrate animal is determined 

 by a common growth-controlling mechanism which is 

 resident in a system of small but complex glandular organs." 

 Some of the modifications effected are the result of 

 abnormal conditions in these glands, initiated by an 

 injury or by disease, the stimulus being derived from the 

 environment. It seems probable that food may indirectly 

 play an important part in stimulating or inhabiting the 

 functions of these glands. According to W. W. Swingles 

 experiments, metamorphosis in tadpoles is accelerated 

 when they are fed with iodine, the th\Toid being enlarged 

 Dr. J. T. Cunningham has expressed the view that external 

 stimulations may effect the genital cells through the 

 chemical influences of these glands, "so as to produce some 

 hereditary effect in succeeding generations."'! 



The origin of variationis — the fundamental problem of 

 evolution — is thus no longer wrapped in complete mystery. 

 It seems that the vitalists are being gradually driven to 

 restricted areas in the fields of biolog}^ Samuel Butler 

 .said that all progress is based upon a universal desire on 

 the part of every organism to live beyond its income. But 

 it is surel}' the result of organisms living in an environment 

 which supplies them with an income in excess of their needs. 

 One thing is obvious : the less complex the environment 

 the more uniform is the fauna and flora, whether we study 

 the present or the past. The march of life through the 

 long ages has developed in consonance with the growing 

 complexity of environments, from the simplicity of 

 ifOllurian conditions in the earliest geological periods on 

 to the diversity of to-day. 



*A. Keith, 'Mature," Nov. 13, 1919. 

 tJ. T. Cunningham, P.Z.S., 1908, p. 434. 



