154 THE KATE OF GROWTH [ch. 



We may summarise, as follows, the main results of the fore- 

 going discussion : 



(1) Except in certain minute organisms and minute parts of 

 organisms, whose form is due to the direct action of molecular 

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

 of growth," or a direct expression of a rate of growth which varies 

 according to its different directions. 



(2) Rate of growth is subject to definite laws, and the 

 velocities in different directions tend to maintain a ratio which is 

 more or less constant for each specific organism ; and to this 

 regularity 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 or fluctuate in a 

 regular way ; and to these progressive changes are due the 

 changes of form which accompany "development," and the slower 

 changes of form which continue perceptibly in after life. 



(4) The rate of growth is a function of the age of the organism , 

 it has a maximum somewhat early in life, after which epoch of 

 maximum it slowly declines. 



(5) The rate of growth is directly affected by temperature, 

 and by other physical conditions. 



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

 retardation, at certain physiological epochs of life, 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 accompaniment 

 of old age. 



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

 temporary increase in the rate of growth (or "acceleration'' of 

 growth) of the injured surface ; in other respects, regenerative 

 growth is similar to ordinary growth in all its essential phenomena. 



In this discussion of grbwth, we have left out of account a 

 vast number of processes, or phenomena, by which, in the physio- 

 logical mechanism of the body, growth is effected and controlled. 

 We have dealt with growth in its relation to magnitude, and to 



