240 



PROBLEMS OF RELATIVE GROWTH 



grasped that most examples of recapitulation constitute 

 simply one side of a more general problem — the problem of 

 altering the relative rates of growth and of other processes 

 within the body. 



t 



t 

 S 



I 1 



X 



time 



Fig. 104. — Diagram to illustrate positive and negative mutations in rate- 

 factors, leading to recapitulation and paedomorphosis respectively. 



X — X, period after which no further differentiation of the character is possible (adult phase in 

 forms with fixed adult size ; maximum size in forms with continuous growth). The curves represent 

 the development of a character controlled by a rate-gene. The portions of the curves to the right 

 of X — X (dotted) cannot be realized. I, condition in ancestral form; II, result of a mutation 

 accelerating the process. The old adult condition (A) now becomes the juvenile condition (J'). 

 A portion of the curve hitherto unrealized is now incorporated in the life-history (heavy line) leading 

 to a new adult condition (A'). Ill, result of a mutation slowing down the process. The old juvenile 

 condition (J) now becomes the adult condition (A"). A portion of the curve (heavy dotted line), 

 including the old adult condition (a), is now extruded from the life-cycle by retardation. 



§ 7. General Approach to the Problem of Qualitative 



Form-change 



In general, it is clear that the effect of our knowledge of rate- 

 factors and of the rules regulating relative growth will make 

 it possible to analyse the problems involved in changes in 

 proportion with a new insight. 



In studying the growth of any part, it is by no means suffi- 

 cient to determine its percentage change in relative size, as 

 has often been the case in the past. A percentage change is 

 the easiest to visualize by graphic methods, and is often very 

 important in comparing related forms. But from the point 



