940 ORIGIN OF SPECIES. 



animals of considerable size perform this ofBce involuntarily, the hooked or rough 

 fruits becoming attached to them and afterwards falling off ^ 



In most of these adaptations, both their purpose and the mechanical con- 

 trivances for its attainment are easily recognised; but not unfrequently the latter 

 require a closer examination and some reflection in order to understand them. 

 Among many other cases of this kind one only may be mentioned here which 

 any one can easily observe for himself. The fruit of Erodium gruinum and other 

 Geraniaceae'^ splits up into five mericarps each of which has the form of a cone with 

 the apex pointing downwards, containing the seed and bearing above a long awn. 

 When moist this awn is stretched out straight, but if it becomes dry while lying on 

 the ground the outer side of the awn contracts strongly, causing the upper end to 

 describe a sickle-shaped curve, which brings its point against the ground, the cone 

 being thus placed with its apex downwards. The lower part of the awn now begins 

 to contract into narrow spiral coils, causing the cone to turn on its axis and to 

 penetrate the ground, and the erect hairs on it which point upwards retain it there 

 like grappling-hooks. After the cone has penetrated the ground, the twisted part 

 of the awn does the same, driving the part which contains the seed further and 

 further into the soil. If the mericarp now becomes moistened, the coiled part 

 attempts to straighten itself, but its coils are held by the hairs which stand on 

 the convex surface ; and thus this movement also contributes to drive the cone 

 deeper into the soil. Whether therefore the moisture is greater or less, the me- 

 chanical contrivance produces the same effect, namely, to drive the part of the 

 mericarp which contains the seed into the soil. 



Some of the contrivances found in plants are extremely striking, from the concur- 

 rence of the most different properties for the attainment of a perfectly definite purpose 

 corresponding only to certain specific conditions of life, as the adaptation of the Virginian 

 Creeper to climbing up vertical walls, the contrivance to prevent self-fertilisation in the 

 flowers of Aristolochia Clematitis, the bursting of the fruit of Momordica Elaterium^ 

 and a thousand similar cases. The most remarkable instances are generally connected 

 with the ordinary arrangements, or even with other extreme cases, by a number of the 

 most diverse intermediate or transitional forms. These transitional forms have been 

 described in detail by Darwin in the case of climbing and twining plants, and the 

 fertilisation of Orchids, in his works already mentioned, and by Hildebrand in the case of 

 the fertilisation of Sal-via^, and of the dissemination of seeds. 



Sect. 39. — The Theory of Descent. The facts and conclusions which have 

 been indicated rather than described are the foundation of the Theory of Descent. 

 This theory consists in the hypothesis that the most unlike forms of plants have a 

 relationship to one another of the same kind as that which the varieties gradually 

 developed from one ancestral form bear to it and to one another. It supposes 

 that the different species of a genus are varieties derived from one progenitor 

 which have undergone further development ; and that in the same manner the various 

 genera of an order owe their common characters to their descent from one and the 



* [A remarkable instance of this is recorded by Dr. Shaw (Journ. Linn. Soc. vol. XIV, 1874, 

 p. 202), in the introduction into South Africa and enormously rapid distribution of a European 

 plant, Xanthium spinosum, by the spiny achenes clinging to the wool of the Merino sheep.] See also 

 Hildebrand, Die Verbreitungsmittel der Pflanzen, Leipzig 1873. 



^ See Hanstein, Sitzungsber. der niederrheinischen Ges. in Bonn, 1868. 



3 Jahrbuch fiir wiss. Bot. vol. IV, 1865. Also Verbreitungsmittel der Pflanzen. 



