THE ORIGIN OF NEW FORMS 199 



It merely determines whether the new form can persist in the 

 place where it arises or to which it may be carried. In adaptation, 

 natural selection and the process of adalptation go hand in hand 

 and selection is obscured. Since an ecad is the direct product 

 of a habitat, it can not arise in a place unfavorable to it, while 

 mutants and hybrids originate regardless of their fitness for the 

 habitat. Natural selection consequently plays no part in pro- 

 ducing new forms by either of these two methods of origin. On 

 the other hand, it furnishes the only means of accumulating and 

 preserving minute indefinite variations, and it plays a part in 

 adaptation. The question of its value in evolution must rest 

 chiefly upon experimental evidence that new forms originate by 

 variation. 



212. Isolation. As already indicated, the fate of any new form 

 is determined not only by competition and the physical factors 

 of the habitat; it depends also upon the chances of crossing with 

 the parent form or related forms. If forms do arise by variation 

 and selection, the possibility of origin depends largely upon the 

 absence of repeated inter-crossing. Forms which are prevented 

 from crossing with each other are said to be isolated. Isolation 

 may be due to physical or biological barriers, to distance, or in 

 short to any condition which prevents the access of strange pollen. 



Isolation has often been thought a necessary condition for the 

 origin of species. This can be true only of such forms as originate 

 through variation. Minute differences between the various in- 

 dividuals would be constantly leveled, and the cumulative action 

 of selection would be effective only upon individuals cut off from 

 the main group. In origin by adaptation and mutation, isolation 

 naturally can take no part. Yet it does directly affect the per- 

 sistence of either ecad or mutant by preventing crossing and 

 consequent merging with the parent form. In so far as origin by 

 hybridation is concerned, it is evident that isolation renders it 

 impossiljle, owing to its complete dependence upon cross-pollination 

 as a cause. 



213. Polygenesis. The origin of a form at two or more distinct 

 places or times is known as -polygenesis. Species were long sup- 

 posed to have been created but once and in a single place. This 

 idea of a single origin for all species was carried over into evolution, 

 and Danvin maintained it vigorously against the doctrine of 

 multiple origin. For many years after the general acceptance 



