14 ELEMENTS OF PALAEONTOLOGY 



(liyper-tropliic) peculiarities, or too high specialisation of organs, is as a rule 

 injurious to the form and leads usually to its extermination. Many groups 

 remarkable for their extreme differentiation (Dinosauria, Pterosauria, AmUypoda, 

 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. 



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

 animals (Blastoids, Tetracomlla, Trilobites, Ammonites, Rudistae, Ichthyosauria, 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, and the encroachments of natural enemies, may have led in many 

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

 to account for the disappearance of an entire species or particular group 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 dotage, they evince all 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, supposing that the palaeontological 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 hundreds 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 rocks by petrographical changes, a greater or lesser number of species 

 either entirely disappears, or is replaced by closely related, but at the same 

 time more or less different forms. Obviously, therefore, there have been 

 periods when the process of transformation and the weeding out of organisms 

 were greatly accelerated, and following upon these reconstructive periods long 

 intervals of repose have ensued, during which intervals species have adhered 

 to their characteristic forms with but little variation. The fact that evolution 

 has advanced by occasional bounds or leaps stands, however, in nowise contra- 

 dictory to the theory of descent. 



The whole animate community at any point on the earth's surface rests 

 normally in a state of equilibrium, the balance being maintained by the con- 

 certed action of all ranks and members of society. For the preservation of 

 this balance nature practises a most rigid domestic economy. Every plant 

 depends upon particular conditions of soil, food, temperature, moisture, and 



