14 ELEMENTS OF PALAEONTOLOGY 
(hypertrophic) 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, Amblypoda, 
Toxodontia, ete.) 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, Tetracoralla, Trilobites, Ammonites, Rudistae, Ichthyosauria, ete.) 
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 
