GEOGRAPHY AND PALAEONTOLOGY
THE forms assumed by living matter are countless, and vary, as we have seen, in both space and time. They are not identical at different places on the planet nor in different geological periods.
We have seen that each region of the earth, in the seas or on the land, has its own characteristic fauna and flora. The species living in any particular region are determined by a multitude of circumstances, for instance, humidity, temperature, particular foodstuffs available, and other co-existing and competing types of life, resulting in a balance between the various creatures which live contemporaneously or succeed one another. Thus it is that each country is characterized by its flora and fauna.
The areas of diffusion of the various species are also defined according to the circumstances obtaining in them. Plants generally show a more limited distribution, because it is impossible for them to move about. On the other hand, animals, being possessed of the power of movement, are able to wander to greater or less distances and spread. Even so, there are differences among the animals also, particularly observable in fossils, where we find deep-sea sedentary types or benthos living on the ocean bed more than a thousand fathoms down and usually showing themselves only in circumscribed areas which sometimes help us to define a particular geological facies or period of the earth's history. Wandering creatures, as for instance nekton and plankton, have a wider habitat.
Fortuitous chances of geography may impede or, on the contrary, foster the spreading of a species. Such particularly is the effect of mountains, streams and oceans. A species at a particular place tends to propagate itself, and the success or failure of its efforts will depend on its own special qualities, which include, for instance, greater or less incentive to live, adaptability, strength, reproductive powers, and so on, whilst on the other hand environmental conditions, be they avourable or adverse to the species, will also play their part. Among environmental conditions we must place foremost the possible