944 ORIGIN OF SPECIES. 



their varieties, and how long it takes for these new properties to become hereditary, 

 and further how enormous is the diversity of hereditary properties, we are forced to 

 the conclusion that an inconceivably long period must have elapsed since the 

 appearance of the first plants on the earth. But geology and the physical nature of 

 the globe require as great a space of time for the explanation of other facts ; and a 

 few millions of years more or less is a matter of but little consequence in the expla- 

 nation of facts which require lapse of time in order to reach a given magnitude. 



The first rudiments of the Theory of Descent, which holds good for the animal as for 

 the vegetable kingdom, may be traced to Lamarck, at the commencement of the century, 

 in his Zoologie Philosophique (Paris, 1801); it was afterwards advocated by Geoffroy 

 St. Hilaire ; but it is only since the publication of Darwin's work * On the Origin of 

 Species by means of Natural Selection' (London, 1859) that it has become an integral 

 part of science. Darwin's great service to science is to have established as a fact the 

 struggle for existence which all living beings have to fight, and to have proved its action 

 in the maintenance or destruction of new forms. It is only by means of the struggle for 

 existence that the motive principle is recognised, and that the theory of descent is enabled 

 to solve the great problem why parts which are morphologically similar are adapted for 

 such different functions ; and conversely also to show how purpose in organisation, and at 

 the same time the relations of affinity among plants, can be explained. Darwin considers 

 the Natural Selection which the struggle for existence brings about as the sole cause 

 of the increasing differentiation of plants which are undergoing variation ; he starts with 

 the hypothesis that every plant varies in all directions without any definite tendency to 

 become further developed in any one particular direction. He attributes to the struggle 

 for existence alone the power of securing the perpetuation of one or more varieties 

 among the countless numbers which are produced, and is convinced that in this way not 

 only is a perfect adaptation of the new forms effected, but morphological differentiation 

 is also carried further. Nageli ^ assumes, on the contrary, that each plant has in itself 

 a tendency to vary in a definite direction, to increase the morphological differentiation, 

 or, as it is commonly expressed, to perfect itself. The great differences of a purely 

 morphological nature between the classes and smaller divisions of the vegetable kingdom 

 may then owe their existence to this internal tendency towards a higher and more varied 

 differentiation ; while the struggle for existence brings about the adaptation of the 

 separate forms. The great services of the theory of descent remain in either case. 



In all future research it will be of primary importance to distinguish clearly between 

 those peculiarities of plants which have no reference to the external world, which are, 

 that is, purely morphological, and those without which it would be impossible for the 

 plant to continue to exist under certain external conditions. It appears to be certain 

 that the latter are only produced by adaptation in the struggle for existence^. 



The first and simplest plants had no ancestors ; they arose by spontaneous generation. 

 Whether this took place only once ; whether only one or a number of primitive plants 

 were produced simultaneously, giving origin in the latter case to diflferent developmental 

 series, or whether, as Nageli supposes, spontaneous generation has taken place at all 

 times, and is now taking place, giving rise to new developmental series, are questions 

 which still await solution, and which we cannot follow out further here. 



* Nageli, Entstehung und BegrifFder naturhistorischen Art, Munich 1865. 



^ Pringsheim has recently drawn attention to certain phenomena in the Sphacelarise which 

 show that a continued development with increasing morphological differentiation may take place 

 independently of the struggle for existence (Abh. Berl. Acad. 1873). Cultivated plants show, on 

 the other hand, that a considerable accumulation of new physiological properties may take place 

 unaccompanied by important morphological changes. 



