Ill] SUMMARY AND CONCLUSION 283 



of a drop of water) is due to the direct action of the molecular forces, 

 we may look upon the form of an organism as a " function of growth," 

 or a direct consequence of growth whose rate varies in its different 

 directions. In a newer language we might call the form of an 

 organism an "event in space-time," and not merely a "configuration 

 in space." 



(2) Growth varies in rate in an orderly way, or is subject, like 

 other physiological activities, to definite "laws." The rates differ 

 in degree, or form "gradients," from one point of an organism to 

 another; the rates in different parts and in different directions 

 tend to maintain more or less constant ratios to one another in 

 each organism ; , and to the regularity and constancy of these relative 

 rates of growth is due the fact that the form of the organism is in 

 general regular and constant. 



(3) Nevertheless, the ratio of velocities in different directions is 

 not absolutely constant, but tends to alter in course of time, or to 

 fluctuate in an orderly way; and to these progressive changes are 

 due the changes of form which accompany development, and the 

 slower changes which continue perceptibly in after hfe. 



(4) Rate of growth depends on the age of the organism. It has 

 a maximum somewhat early in hfe, after which epoch of maximum 

 it slowly declines. 



(5) Rate of growth is directly affected by temperature, and by 

 other physical conditions: the influence of temperature being 

 notably large in the case of cold-blooded or " poecilothermic " 

 animals. Growth tends in these latter to be asymptotic, becoming 

 slower but never ending with old age. 



(6) It is markedly affected, in the way of acceleration or retarda- 

 tion, at certain physiological epochs of hfe, such as birth, puberty 

 or metamorphosis. 



(7) Under certain circumstances, growth may be negative, the 

 organism growing smaller; and such negative growth is a common 

 accompaniment of metamorphosis, and a frequent concomitant of 

 old age. 



(8) The phenomenon of regeneration is associated with a large 

 transitory increase in the rate of growth (or acceleration of growth) 

 in the region of injury; in other respects regenerative growth is 

 similar to ordinary growth in all its essential phenomena. 



