C— GEOLOGY 65 



Study of the ' survival-value ' of various types in groups whose palae- 

 ontological history is adequately known reveals many points of interest. 

 Every group includes some types that are relatively persistent and others 

 that are relatively ephemeral. For example, among the sea-urchins, 

 Cidaris has persisted with no important modification from the Triassic 

 period to the present day ; the family of the Cidaridae ranges back to the 

 Carboniferous period . Echinocystis, a sea-urchin that appeared long before 

 any of the Cidarida;, was limited in range to the Upper Silurian. Hetero- 

 salenia, appearing first in the Upper Jurassic, disappeared in the Upper 

 Cretaceous. Now Echinocystis and Heterosalenia were both much more 

 elaborate in structure than Cidaris, so that their short ranges illustrate the 

 generalisation made above. But Bothriocidaris, an early Echinoid far 

 simpler in structure than Cidaris, appeared and became extinct within 

 the Ordovician period. A closely parallel series of cases could be cited 

 among Brachiopoda or Mollusca. In these groups the persistent genera 

 Lingida, Nncula and Patella were neither the earliest to appear nor the 

 simplest in structure. They represent, however, like Cidaris, the simplest 

 types capable of living with a fair measure of efficiency in the circumstances 

 appropriate to their kind. Such types never attain the temporary import- 

 ance often reached by highly specialised types ; they remain compara- 

 tively obscure members of the fauna : but they remain. No imagination 

 is needed to see in a limpet the modern representative of a type that was 

 ancient before the first Vertebrate appeared. A trace of poetic insight 

 would show that its humility has been its salvation. 



The harmony that exists between the structures of organisms and their 

 environment would be incredible were it not commonplace. But the 

 explanation of that harmony is not yet available, although from the days 

 of teleology to the present it has been the ultimate aim of most biological 

 research. Do organisms endowed with certain structures deliberately 

 select suitable environments (as a Red-underwing moth chooses an elm- 

 bole as a resting-place), or does the environment impress on, or extract 

 from, the organism appropriate reactions (as the grime of a city seems 

 to induce melanism) ? Even to-day the only safe reply to this question is 

 to repeat another about a hen and an egg. 



Nevertheless, in one aspect of the question there is definite evidence. 

 On individual organisms environment can at least exert the power of 

 a final veto. Environment the executioner is so potent in individual life 

 that it may, indeed it must, accelerate the extinction of any series of 

 organisms whose structures fail to conform to its requirements. A con- 

 stant environment is a sure means of maintaining constancy in the char- 

 acters of successive generations ; any deviation from the permitted pattern 

 cannot fail to prove less perfectly attuned than the orthodox plan. In 

 geological time, however, environment is sure to change, so that a group 

 of organisms will inevitably drop behind the times unless it can adjust 

 its characters or its distribution to the shifting demands of its surroundings. 



Ample evidence of the soundness of this argument can be found in 

 Palaeontology. Although groups of organisms may become extinct at 



