846 • ORIGIN OF SPECIES. 



When we reflect ^vhat a number of generations our cultivated plants must have 

 passed through before any considerable amount of new properties were manifested in 

 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 in 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, to show how purpose in organisation can be ex- 

 plained, and at the same time the relations of affinity among plants, Darwin considers 

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

 the increasing diff'erentiation 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. Niigeli^ assumes, on the contrary, that each plant has in itself 

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

 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 

 can 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. Weighty arguments can be brought forward for and against this theory 

 of Nageli's ; but in the present state of science I think it is impossible to decide either 

 way; the great services of the theory of descent remain in either case; Nageli's view 

 does not exclude Darwin's ; but the latter includes the former as a more special case. 



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

 or special creation. W^hether this took place only once ; whether only one or a number 

 of primitive plants were produced simultaneously, giving origin in the latter case to 

 different series of development, or whether, as Nageli supposes, spontaneous generation 

 has taken place at all times, and is now taking place, giving rise to new series of de- 

 velopment, are questions which still await solution, and which we cannot follow out 

 further here. 



Nageli, Entstehung und Begriff der naturhistorischen Art ; Munich, 



