842 



ORIGIN OF SPECIES. 



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 qualities for the attainment of a perfectly definite purpose 

 corresponding only to certain specific vital conditions, 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 structures. The most beautiful instances are generally connected with 

 the ordinary structure, 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 fertilis- 

 ation of Salvia^. 



Sect. 37. — 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 

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

 by their descendants of new characters in the course of a long series of generations. 

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

 between the various orders of a class, and finally between the various groups. 

 It considers variation with descent to be the cause of all the differences among 

 plants; and the inheritance of these characters to be the cause of the agreement 

 which subsists even between 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 descended from one ancestral form or Archetype, as 

 Darwin terms it. That which was long since termed in a merely metaphorical sense 



Jahrbuch fiir wiss. Bot. vol. IV, 1865. 



