C— GEOLOGY 67 



of conditions can be appropriate to simple structures than to complex. 

 Indeed, we may go further, and conclude that simplicity implies catholicity 

 and complexity implies specialisation. A simple type, with simple needs, 

 is long-suffering under change ; a complex type, with peculiar needs, is 

 distraught if those needs are not met in their entirety. A Jack-of-all- 

 trades has a better prospect of finding a job than a specialist. 



This principle, while explaining the longevity of simple types, can 

 only explain the shortness of the careers of complex types if we assume 

 that such types are incapable of modification consonant with changes 

 of environment. Although there are very many cases where a stereo- 

 typing of structure has undoubtedly had a fatal sequel for this very reason, 

 there are also cases where highly elaborate types have come through 

 physiographical crises unharmed. One of the most notable of these 

 cases is found in the Ammonoidea. The Permo-Carboniferous members of 

 that group were at least as complex in structure as any before or since 

 that time, but the Hercynian revolution had little or no effect upon their 

 quality or dominance. Their success is made the more dramatic by the 

 spectacular collapse at about that time of the Nautiloidea, an allied group 

 with much the same habits of life. Evidently complexity is not necessarily 

 fatal, although it is more dangerous than simplicity. 



In an endeavour to find an explanation for the patent fact of varying 

 reaction to environment, recognition of the principle of evolution becomes 

 inevitable. If all types were irrevocably fixed in character, the meek 

 would long ago have inherited the earth ; all complex and specialised 

 types would have met their doom during the succession of geological 

 changes. But in fact, although a steady undercurrent of simple types 

 flows unchecked through the record of palaeontological history, the 

 frequent and spectacular disasters, like the bursting of bubbles, that have 

 befallen the complex types have but opened the way for others of equal 

 complexity to rise to the surface. 



One of the most stimulating glimpses into the mode of evolution was 

 given by the work of Alpheus Hyatt and his successors, notably C. E. 

 Beecher and R. T. Jackson. The main thesis of their interpretation con- 

 sists of a kind of extension of the neontological theory of recapitulation to 

 fossil forms. When recapitulation was found to continue after the em- 

 bryonic or larval stages, and to persist throughout the life of an individual, 

 a much more satisfactory element was brought into the theory. Larval 

 stages are often passed under conditions that could never have been 

 tolerated by the adult forms that they are supposed to recall ; whereas there 

 is no reason why an adolescent or adult individual should not occupy an 

 environment similar to that of its ancestors. Moreover, the relatively 

 slow rate of growth and development after the larval stage makes the 

 discrepancy between the speed of evolution and that of ontogeny less 

 intense. 



By application of this principle, especially to the cases of Ammonites 



