CERTAIN LAWS OF VARIATION. 201 



elucidation of the law in question, though they are in- 

 sufficient to afford an absolute criterion of its extent. 

 Thus it is obvious that the reaction of a growing organ- 

 ism to its environment must depend on its rate of 

 growth at the time. Supposing that in the earliest 

 stages of development it doubles its weight within a 

 day, whilst in the later stages only within a month, then 

 an unfavourable environmental condition, acting for a 

 given time during the early period, may conceivably 

 produce thirty times more retardation of growth than 

 when acting during the later period. To what degree 

 retardations of growth produced at various stages of 

 development persist to the adult stage is another ques- 

 tion, and reference will be made to it later on. 



In the case of man, the data relative to the embryonic 

 rate of growth are not very reliable, but they are quite 

 sufficiently so for our purpose. A considerable number 

 of them have been collected by Preyer.* His has fur- 

 nished data as to the length of embryos during the first 

 few weeks of development, and Hennig has given a table 

 of measurements for the 1st to the 10th month, these 

 being averages derived from a hundred observations. 

 As Minot points out,t however, these data are inexact, 

 for in the early stages only the head and trunk were 

 measured, and in the later stages head, trunk, and legs. 

 In human embryos weight is a better criterion of de- 

 velopment than length, but as in the observations of 

 His only the length was determined, it is necessary, for 

 purposes of comparison, to keep to this standard 

 throughout. In order to obtain readily comparable 



* " Specielle Physiologic des Embryo," p. 500, Leipsig, 1885. 

 f " Human Embryology," p. 336, New York, 1892. 



