But now let it be observed that the revolution thus resulting would not be a substitution of a thousand more or less modified species for the thousand original species; but in place of the thousand original species there would arise several thousand species, or varieties, or changed forms.  Each species being distributed over an area of some extent, and tending continually to colonise the new area exposed, its different members would be subject to different sets of changes.  Plants and animals spreading towards the equator would not be affected in the same way with others spreading from it.  Those spreading towards the new shores would undergo changes unlike the changes undergone by those spreading into the mountains.  Thus, each original race of organisms, would become the root from which diverged several races differing more or less from it and from each other; and while some of these might subsequently disappear, probably more than one would survive in the next geologic period:  the very dispersion itself increasing the chances of survival.  Not only would there be certain modifications thus caused by change of physical conditions and food, but also in some cases other modifications caused by change of habit.  The fauna of each island, peopling, step by step, the newly-raised tracts, would eventually come in contact with the faunas of other islands; and some members of these other faunas would be unlike any creatures before seen.  Herbivores meeting with new beasts of prey, would, in some cases, be led into modes of defence or escape differing from those previously used; and simultaneously the beasts of prey would modify their modes of pursuit and attack.  We know that when circumstances demand it, such changes of habit do take place in animals; and we know that if the new habits become the dominant ones, they must eventually in some degree alter the organisation.

Observe, now, however, a further consequence.  There must arise not simply a tendency towards the differentiation of each race of organisms into several races; but also a tendency to the occasional production of a somewhat higher organism.  Taken in the mass, these divergent varieties which have been caused by fresh physical conditions and habits of life, will exhibit changes quite indefinite in kind and degree; and changes that do not necessarily constitute an advance.  Probably in most cases the modified type will be neither more nor less heterogeneous than the original one.  In some cases the habits of life adopted being simpler than before, a less heterogeneous structure will result:  there will be a retrogradation.  But it must now and then occur, that some division of a species, falling into circumstances which give it rather more complex experiences, and demand actions somewhat more involved, will have certain of its organs further differentiated in proportionately small degrees,—­will become slightly more heterogeneous.