THEORY OF DESCENT, 843 



the affinity between different forms of plants is, according to the theory of descent, 

 an actual affinity or blood*relationship 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 ; and the different individuals varying in different ways, 

 the difference between them continually increases, and must continue to increase 

 under diverse conditions of climate, and especially under those imposed by the 

 struggle for existence, in order that they may still be capable of maintaining them- 

 selves. At the same time numberless varieties, species, and genera are constantly 

 disappearing, because they are 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 are better adapted to resist it. 



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. ; since it requires no other 

 hypothesis than descent with variation and the continued struggle for existence which 

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

 periies, the others perishing sooner or later. But both these hypotheses are sup- 

 ported 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 resisting 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 

 Apocyniim androscEmifolium by flies which are tortured to death in the process). 



The fact that members which are morphologically similar are adapted for the 

 most various functions is explained when we consider that the morphological 

 features in the structure of plants are those which are most certainly transmitted 

 unchanged to posterity, either because they are useless in the struggle for existence, 

 or because they have proved useful in the various relations of life ; as for example 

 the differentiation into stem, root, leaves, &c., and into the different tissue-systems^ 

 by which the division of physiological labour and the acquisition of the most various 

 properties useful for the struggle for existence are facilitated. The structure of 

 Thallophytes, Characeae, and Hepaticae, shows that these morphological differentia- 

 tions do not exist in the first or lowest forms of plants, but that they come 

 gradually into existence ; but when once fully developed they are preserved by 



