276 PKOCEEDINGS OF THE MA1A.C0L0GICAL SOCIETY. 



itself, and possibly to a lesser degree other agencies, by their action 

 on the organism call forth such variation. 



Natural Selection then comes into play, and working on the 

 varieties which the environmental conditions have provoked and 

 continue to stimulate, determines, through the survival of the fittest, 

 which of these varieties shall develop into new species. 



The process is a consistent whole, and it is waste of time and logic 

 to argue wiiether the actual origin of the species is to be counted from 

 the time of intervention of natural selection, or reckoned from the 

 point when incipient variation first showed itself. All organisms 

 have undergone and are still undergoing this process of evolution, of 

 which we know not the beginning and cannot forecast the end. 



Of all the divisions of tlie Animal Kingdom the Mollusca probably 

 furnish the best means of tracing out these workings of evolution, for 

 unlike the higher organisms, whose parts only teach the condition of 

 the individual at the moment, the shell of the mollusc properly 

 dissected will yield evidence of its whole life-history. This being 

 true of fossil equally with recent forms, they offer a fine fi^eld for 

 investigation. So, too, in a lesser degree, do the Brachiopods and 

 Corals. It may not, therefore, be out of place on the present occasion 

 to summarize what has so far been done in this branch of research, 

 and to indicate what further opportunities for investigation are open. 



Hyatt was one of the first to seize on the evolutionary idea and 

 apply it to his particular study, the fossil Cephalopoda. He demon- 

 strated that each Ammonite (and less conspicuouslj^ each Nautiloid), 

 when broken up and examined, could be shown to pass through a series 

 of stages changing its form with growth ('ontogenesis'). So great 

 sometimes is the difference between the earlier and later stages that 

 it has not uncommonly happened for two stages of growth in the 

 same Ammonite to have received distinct specific names. For these 

 stages he proposed terms (39) which, as subsequently modified in 

 accordance with suggestions made by Mr. S. S. Buckman & Dr. Bather 

 {13), have obtained wide currency inasmuch as they are applicable to 

 all forms of animal life (^1, 1^.2). 



For the sake of those to whom these terms may not j^et be familiar, 

 it is permissible to recapitulate them here ; they are — 



1. Embryonic. 



2. Nepionic . . Larval, or young. 



3. Neanic . . Immature, or adolescent. 



4. Ephebic . . Mature, or adult. 



5. Grerontic . . Senile, or old. 



When these terms are applied to the race instead of the individual 

 the root ' phylo ' is prefixed. 



Hj^'att's next interesting point ('phylogenesis') was that the earlier 

 stages in each individual resembled the adult stages of forms which 

 immediately preceded them in geological time, and of which they were 

 the modified descendants {3lj.). Thus in some Ammonites the young 

 shell is smooth, and the margins of the simple septa show but slightly 

 sinuous sutures where they join the shell-wall — in effect they bear 



