INTRODUCTION 15 



leads usually to its extermination. Many groups remarkable for their 

 extreme diflerentiation (Dinosauria, Pterosauria, Amblypoda, Toxodontia, 

 etc.) have become extinct probably for this reason, since, having advanced so 

 far in a single limited direction, adaptation in other directions was no longer 

 possible. 



Persistent types seldom produce a large number of species during a single 

 geological period ; types that start up suddenly and proceed to vary rapidly 

 as a rule soon die out ; while groups that develop slowly and steadily usually 

 contain in their growth the promise of great longevity. 



Some very ancient types have persisted to the present day in highly 

 saline lakes, or in salt pans, in acid, alkaline, very cold or otherwise unnatural 

 situations. These represent dominant types of past ages which, vigorous and 

 adaptable, when forced by internal specific pressure due to the enormous 

 increase in the numbers of individuals were able to invade and adapt them- 

 selves to physically and chemically unfavourable localities. The subsequent 

 development of other types, younger and more vigorous, has extirpated them 

 from all the more desirable situations, though these types have not proved 

 sufficiently adaptable or vigorous entirely to exterminate them. 



The fauna of the ocean deeps and of biologicall}^ unfavourable situations 

 generally is, therefore, a curious composite of the more vigorous and adapt- 

 able types of animal life from the Cambrian to the present day, including 

 forms which were dominant in all earlier epochs, as well as forms derived directly 

 from recent ancestors. 



For the extinction of many plants (Sigillaria, Lepidodendron, Cordaites) and 

 animals (Blastoids, Tetracoralla, Trilobites, Ammonites, Rudistae, Ichthyosaurs, 

 etc.) of former periods no adequate explanation has as yet been found. Changes 

 in external conditions, especially such as regards the distribution of land and 

 water, climatal conditions, saltness of the water, volcanic eruptions, paucity of 

 food-supply, the encroachments of natural enemies, and diseases, may have 

 led to the extinction of certain forms, but such conjectures signally fail to 

 account for the disappearance of an entire species or particular groups of 

 organisms. Oftentimes extinction seems to have been caused merely by 

 superannuation. Long-lived forms belong for the most part to persistent 

 types whose range of species is limited. Their reproductive functions have 

 declined, and like an individual in its senescence, they evince the symptoms of 

 decrepitude and old age. Darwin attributes the extinction of less well-adapted 

 organisms to the struggle for existence ; but since, according to the theory of 

 natural selection, new species arise only with extreme slowness by means of 

 the gradual accumulation of useful variations, and since in like manner their 

 less successful competitors are only very gradually crowded out, we should 

 expect to find in the rocks, siipposing that the paleontological record were in 

 any degree perfect, all manner of extinct intermediate forms, and we should be 

 able, at least for those groups especially liable to conservation, to build up 

 complete ancestral trees. But as observation shows, not only do most plants 

 and animals now living in a wild state adhere to their peculiar characteristics 

 with great tenacity, exhibiting barely appreciable changes even in the course 

 of himdreds or thousands of years, but, furthermore, fossil species remain 

 within the limits of a single geological period fairly constant. With the 

 beginning of a new epoch or period, however, which is usually indicated 

 in the section by lithologic changes, a greater or less number of species either 



