176 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



processes such as mutation (including chromosomal multiplication 

 or other change), hybridization (producing new combinations of 

 genie materials), isolation, and natural selection, so does evolution 

 regardless of form result in particular physiological make-up. Here 

 are included those physiological factors on which depend the suit- 

 abilities of particular plants to grow in particular areas. 



By such means is delimited the potential area of a species, outside 

 of which it cannot grow naturally. The ultimate limits are accord- 

 ingly genetically controlled, being restricted primarily to areas of a 

 particular climatic range and secondarily, within these, to areas of 

 particular sorts of soils, etc. And even though acclimatization 

 allows some latitude, this can only be within genetically fixed limits 

 — unless, of course, there is some fundamental evolutionary change 

 in the race. This sometimes happens, for example through muta- 

 tion or hybridization. Also effective in this direction is some change 

 in the population, such as can take place through isolation and 

 selection of the forms best adapted to withstand particular local 

 conditions. The latter type of instance is not so much genetically 

 controlled as genetically allowed, for, as we have already seen, a 

 population even of a single species or lower entity usually consists 

 of more or less numerous biotypes differing slightly in their inherit- 

 ance. Owing to different groupings of biotypes occurring in 

 different local populations, or at all events to varying selection, 

 different geographical races of a species often appear that have 

 different habitat preferences and, consequently, ranges. And geo- 

 graphical isolation promotes evolutionary divergence — not merely 

 because of differing selection-pressures and variational tendencies, 

 but also because mutations appearing subsequently cannot be shared 

 around by interbreeding ; in time, new species may result. 



In spite of this relative ease and speed of evolution in some 

 instances, it appears to be extremely slow in others. Thus it is 

 supposed that most at least of the woody species of temperate 

 regions date well back into the Tertiary, having existed for five 

 million or more years. That, as we have seen, was a time when 

 equable climates extended much farther towards the poles than they 

 do nowadays, and woody species of temperate regions tended to be 

 much more widespread. One of the most active principles tending 

 to blur the immediate effects of evolution is introgression, which 

 is the gradual infiltration of the germ-plasm (and hence inculcation 

 of facets of the character) of one species into that of another as a 

 result of hybridization and repeated back-crossing with the original 



