THEORY OF DESCENT. 



941 



same older ancestral form, and their differences to variation and to the accumulation 

 )f new characters in successive individuals through a long series of generations. 

 'he theory of descent goes still further, and assumes the same mutual relationship 

 itween the various orders of a class, between the various classes of a group, and 

 inally between the various groups. It considers variation in the course of repro- 

 'duction to be the cause of all the differences among plants, and the heredity of 

 the varietal characters to be the cause of the agreement which subsists even be- 

 tween the most diverse forms of plants. What we call the common law of growth 

 of a class, or in other words its Type, is the result of all the plants of this class being 

 iescended from one ancestral form or Archetype, as Darwin terms it. That which 

 ras long since termed in a merely metaphorical sense the affinity between different 

 forms of plants is, according to the theory of descent, an actual affinity or blood- 

 Telationship in various degrees. The differences have arisen in the course of a long 

 series of generations, by the descendants of the same archetype continuing to vary 

 by the variation of the different individuals in different ways, and by the continual 

 and necessary increase of the differences between them under diverse conditions of 

 climate, especially under the conditions imposed by the struggle for existence, in 

 order that they may still be capable of maintaining themselves. At the same time 

 numberless varieties, species, and genera have gradually disappeared, because they 

 were not sufficiently adapted for the struggle for existence under the new conditions 

 caused by geological changes, and in consequence of the appearance of other forms 

 which were better adapted to resist them. 



The scientific basis for the theory of descent rests in the fact that it alone is able 

 to explain in a simple manner all the mutual relationships of plants to one another, 

 to the animal kingdom, and to the facts of geology and palaeontology, their distribu- 

 tion at different times over the surface of the earth, &c. ; for this no other hypothesis 

 is necessary than descent with variation and the continued struggle for existence which 

 permits those forms only to persist that are sufficiently endowed with useful pro- 

 perties, the others perishing sooner or later. Moreover both these hypotheses are 

 supported by an infinite number of facts. The theory of descent involves only one 

 hypothesis that is not directly demonstrated by facts, namely that the amount of 

 variation may increase to any given extent in a sufficiently long time. But since the 

 theory which involves this hypothesis is sufficient to explain the facts of morphology 

 and adaptation, and since these are explained by no other scientific theory, we are 

 justified in making this assumption. 



The theory of descent explains intelligibly how plants have obtained their 

 extraordinarily perfect adaptations for supporting the struggle for existence ; this 

 struggle has itself been the means of their obtaining them by the * Survival of the 

 Fittest,' that is, by permitting the existence and propagation of those newly-formed 

 varieties alone which are endowed with the various characters that render them 

 best fitted to the climate and to resist the rivalry of competitors, the attacks of 

 animals, &c. In this manner adaptations are gradually developed from a slight and 

 imperceptible beginning by the accumulation of useful characters which have the 

 appearance of being the result of the most careful and far-sighted calculation and 

 deliberation, or sometimes even of the most cruel caprice (as in the fertilisation of 

 Apocynum androscemi/olium by flies which are tortured to death in the process). 



