DERIVATION OF EXISTING SPECIES. 595 



every probability that amongst the assemblage of new forms some will be better 

 adapted to the new conditions when a change of climate occurs than those of the old 

 species which are driven out thereby, and that these new forms will therefore be 

 able to take the place of the latter. 



It is only from this standpoint that we can properly understand the phenomena 

 of the alternation of generations, the separation of the sexes, dichogamy, and all 

 the rest of the wonderful floral contrivances, the object of which is to facilitate the 

 crossing of two species during the first stage of flowering and only to allow of 

 crossing between plants of one species, or of geitonogamy, autogamy, or cleisto- 

 gamy in the event of no inter-specific crossing taking place. As a result of these 

 contrivances, numberless new forms are continually being generated which are 

 respectively adapted to all the most various conditions of soil and climate. So 

 long as no change in climatic conditions takes place, the majority of these forms 

 have very little chance of surviving and of naturalizing themselves as species 

 amongst the plants already established in the same locality. But when, in conse- 

 quence of a change of climate, the ranks of the species in possession of the ground 

 are thinned through the abdication of many of those best adapted to the condi- 

 tions of life previously existing, the real significance of the new forms which have 

 arisen as a result of the sexual process is manifested in the fact of those which are 

 best adapted to the new conditions taking possession of the spots vacated and 

 settling down there as new species. 



DERIVATION OF EXISTING SPECIES. 



The plants preserved as fossils in former ages are not only the forerunners 

 but the ancestors of the existing vegetation of to-day. There was no general 

 rejuvenescence and extinction of organisms coincident with the beginning and end 

 of the several "periods" of the history of the earth. The changes in the organic 

 world, like those in the inorganic crust of the earth, were accomplished gradually by 

 slow degrees, and the organisms of the present day are a continuation of, and have 

 been slowly evolved from, those of former ages. 



So far, there is little difference of opinion amongst naturalists; but as to the 

 •causes of the differences in form between the vegetation of the present and the past, 

 the most various theories are held. Nor is this surprising, seeing how largely our 

 eonclusions are based on conjectures. And when the ffood-gates of speculation are 

 rolled back it is not always that the proven is clearly distinguished from the 

 unproven. An import is attached to isolated facts which they do not merit, and — 

 most mischievous of all — the existence of wide lacunee in our knowledge is con- 

 «ealed, or these lacunae are dexterously bridged over with unmeaning, high-sounding 

 words and hollow phrases which, while astonishing us for the moment, leave us 

 chastened and confounded. The confirmed mistrust aroused by these extravagances 

 which obtains concerning all that bears on the derivation of species demands that 

 we should devote a brief consideration to the prevailing theories, and especially to 



