CERTAIN LAWS OF VARIATION. 203 



height of girls and boys between their 5th and 6th 

 years is 105.2 cm., or only just over double the birth 

 length. The rate of growth during this period is there- 

 fore only about 1-2 80th part of that in the third week 

 of embryonic growth. Between the 5th and 10th 

 years it is only about l-900th part, and between the 

 15th and 19th years only l-2400th part. Compared 

 with the calculated rate of growth during the first day 

 of development, the rate during this last period is about 

 5000 times smaller. 



As regards another mammal, the guinea-pig, a very 

 complete series of observations has been made by 

 Minot,* though unfortunately these extend only to the 

 post-embryonic stages. Minot has calculated the daily 

 percentage increase in weight of his animals, and he 

 found that this diminished steadily from the fourth day 

 after birth onwards. During the first week or two it 

 was over 5 per cent, per diem; at the end of the second 

 month about 1.3 per cent., and after a year about .1 

 per cent. Minot seems to have been the first observer 

 to recognise the importance of calculating the relative 

 rate of growth, as distinguished from the absolute in- 

 crements of weight. He considers there can be little 

 doubt that there is a progressive loss of growth power 

 at least in all mammals, and perhaps in all living beings. 



That the rate of growth in the earliest stages of de- 

 velopment is enormously more rapid than in the later 

 ones may therefore be regarded as proven. And it 

 seems to me that it is probably a general rule, with 

 doubtless some exceptions, that the effect of environ- 

 ment on the growth of an organism depends chiefly on 

 *J. Physiol., xii. p. 97. 



