780 PART IV. THE PHYSIOLOGY OF PLANTS. 



The Origin of Species. From the foregoing statements relating 

 to heredity and variation, it is possible to draw conclusions not 

 only as to the maintenance of existing species, but also as to the 

 evolution of new species. 



It is clear that new forms may arise in either of two ways : 

 either, first, as the result of the crossing of distinct varieties or 

 species; many recognized species and varieties are probably of 

 hybrid origin : or, second, as the result of variation. The forms 

 thus produced will exist, for a time at any rate, provided that 

 they are capable of reproducing themselves, either vegetatively 

 (e.g. by tubers, bulbs, etc.), or sexually (by seeds, etc.) But their 

 persistence for any long period depends upon their success in the 

 straggle for existence. They have to compete for their life with 

 their parents and with any other closely similar new forms which 

 may have also been developed : if they are successful, they will 

 persist, and their competitors will, some or all of them, die out. 

 The degree of their success depends essentially upon the degree 

 of their adaptation to the circumstances amidst which they have 

 to live. If they excel any of their competitors in the combination 

 of qualities (such as hardiness, time of flowering or fruiting, fer- 

 tility, etc.) which adapt them to the prevailing conditions of life, 

 they will persist and the others will disappear. This condition 

 of struggle constitutes Natural Selection, for the survivors of the 

 struggle are, as it were, selected by the fact of their survival. 



At first all new forms, however produced, appear as what may 

 be termed varieties of their parent species : but, if they possess 

 the requisite reproductive capacity, and if they are successful in 

 the struggle for existence, they eventually become isolated by the 

 disappearance of many of the allied forms, including perhaps the 

 parent forms, and remain as distinct species. Thus in some genera 

 of flowering plants, the recognised species are few and distinct ; 

 whilst in many genera (e.g. Rubus, Hieracium) the greatest diffi- 

 culty is experienced in distinguishing the species and varieties, 

 because the process of isolation by natural selection has not yet 

 been sufficiently operative. 



23. The Theory of Reproduction. The theory of re- 

 production is extremely simple in connexion with the more 

 primitive modes of vegetative propagation. When, for instance, 

 a unicellular organism divides into two, it is easy to understand 

 that the two new organisms will resemble each other and the 

 parent from which they have sprung, because the parental cell 



