PROCESSES OF EVOLUTION 



243 



The spiral coiling of the Gryphaeas, like most molluscan spirals, is 

 a particular example of "heterogonic growth."^ In heterogonic growth, 

 two organs or parts of an animal grow at different rates (measured by 

 increase relative to the mass which is already present) but their growth 



B 



Zi 



Ni 



w 



Number of Whorls. 



l'/4- 



m 



Fig. 107. The Evolution of Gryphaeas. — On the left are drawings of the left 

 valves of Gryphaeas at successive levels in the Lower Lias formation, to show the 

 increase in coiling from the older types (below) to the more recent (above). 

 At the right are shown the frequency curves of variation in coiling in successive 

 populations during a short part of this period. The different levels in the geological 

 sequence (sub-zones) are given distinguishing names, and the zero lines of the 

 frequency curves have been lifted up to show the level of the population in the 

 sequence of beds. 



(From Swinnerton, after Trueman.) 



rates remain in a constant ratio to one another. In the case of spiral 

 coiUng, the two parts whose growth rates are in a constant proportion 

 are the inner and outer edges of a coil. It is remarkable that this same 

 phenomenon seems to be involved in several cases of evolution in 

 trends. Thus the horns of Titanotheres are heterogonic organs; their 

 growth rate is in constant proportion to that of the body as a whole. 



^ Cf. Huxley 1932. 



