884 LIMITS OF DISTRIBUTION. 



It is of great interest to note that the so-called "petites espfeces" (see p. 581) of a 

 particular genus often suppress and replace one another in adjacent regions and 

 climatic zones. The first idea which suggests itself to one observing this phenomenon 

 is that the differences of form exhibited by these races in neighbouring districts are 

 the direct result of the diverse conditions of soil and climate under which they exist, 

 and in former times this was the general opinion of botanists. Even at the present 

 day many botanists hold the view that when a plant-species reaches a place where 

 the climatic conditions differ from those of the home it has till then inhabited, it is 

 able to adapt itself to the new environment, that such adaptation takes the form of 

 an alteration of form, that the change is inherited by the plant's offspring, and that 

 in this manner new species arise. But the results of experiments made on purpose 

 to determine this matter do not justify any such opinion. No success has attended 

 efforts to bring back various " petites especes " to one and the same form by cultiva- 

 tion under precisely similar conditions, nor has any one of those species undergone 

 the anticipated transformation on being transferred to the external environment 

 which was looked upon as the cause of the variation in question. Either the species 

 subjected to the new conditions succumbed thereto and perished without leaving 

 any offspring, or else it underwent such alterations in form as are usually considered 

 to be indicative of varieties merely (see pp. 508-514). These changes were not 

 inherited by the offspring, and no " petite espece " has ever arisen from a variation 

 caused by properties of soil or climate. Such characteristics as are preserved by 

 heredity, and constitute the essence of a species can therefore only have made their 

 appearance, even in the case of "petites especes", as a result of crossing. Whenever 

 characteristics produced by a cross were in harmony with the climatic conditions of 

 a district, the survival of the form which possessed those characteristics was assured. 

 Such a form would be able to acquire through its offspring an area of distribution 

 exactly co-extensive with the appropriate conditions of soil and climate. The two 

 old species from which the new one sprang may both remain in the neighbourhood; 

 it is, however, possible for one only of them to survive, and it is also conceivable 

 that both should have died out. We must not forget in dealing with this question 

 that the age of most species is much greater than was formerly considered possible, 

 that in the case of the majority of species repeated displacements of the area of 

 habitation have taken place since the species arose, that in the course of these 

 displacements the species which belong to a single original stock, and are therefore 

 allied in respect of the history of their evolution, have often been separated from 

 one another, and that a proportion of them have perished and vanished altogether 

 from the scene. 



So long as two areas of plant-distribution, formed in adjacent zones or regions, 

 do not touch anywhere, intercrossing between the denizens of those areas is very 

 difficult, if not impossible, and even the " petites especes " persist unchanged under 

 such circumstances, and preserve their specific characteristics in their offspring. But 

 even where the areas of distribution adjoin one another, and the distance between 

 their native species constitutes no hindrance to cross-pollination, it is still possible 



