114 GEOLOGICAL BIOLOGY. 



organisms. To distinguish them they are called Natural- 

 history Provinces. We say, for instance, that the natural- 

 history province marking the distribution of flowering plants, 

 differs in its boundaries from the province marking the dis- 

 tribution of fresh-water Mollusca. The reason is apparent 

 when we note that the limiting cause of the distribution is 

 perhaps temperature and climate in one case, and community 

 of fresh-water channels in the other. The boundary of the 

 water-bed of a great river-system is the limiting cause of the 

 distribution of the Mollusca, the conditions of temperature 

 and rainfall that of the plants. 



Normal Adaptation to Conditions of Environment. — We have 

 spoken of distribution as applied to organisms. This term 

 implies that each organism is normally adapted to a certain 

 set of conditions, which is called by the general name En- 

 vironment. Within limits the individual adjusts itself to 

 slight change of the environment, but extreme change of tlie 

 conditions of environment restricts the possible living of the 

 particular organism, and for each particular organism the dis- 

 tribution is supposed to mark the particular extent of differ- 

 ing conditions in which it is normally adapted to live. 



Specific Centre of Distribution and Varieties. — Theoretically, 

 each organism is supposed to be qualified to live under a 

 certain set of conditions, and to adapt itself to change of 

 those conditions to a greater or less extent. While geolo- 

 gists do not find a species to be determined rigidly by any 

 one criterion, general usage applies the name Species to those 

 plants or animals which possess common morphological char- 

 acters, and are confined in their distribution to one natural- 

 history province (but taking this as a general definition, ex- 

 ceptions are recognized in the case of species distributed over 

 two or many provinces). Practically, too, each species ap- 

 pears to have a centre of distribution, at which point (or 

 specific centre) the combination of environing conditions are 

 the more favorable; the species may be distributed from this 

 centre, but it is not so abundant outside, and is often seen to 

 present slight differences of form, size, color, or minor differ- 

 ences on the outskirts of the province of its distribution. 

 These differences from the typical form at the centre consti- 



