900 THE EXTINCTION OF SPECIES. 



Botanists of the old school supposed the reason of this to be that the species of 

 Ruhus vary from some unknown cause — presumably from an intrinsic tendency in 

 themselves. At the present day no intelligent observer doubts that many of the 

 plants thus set down as the results of mere spontaneous variation are species which 

 arose by inter-crossing in comparatively early times. Such inter-crossing was 

 rendered possible by the fact that in the course of those displacements and altera- 

 tions affecting floras, to which the present distribution of plants must be attributed, 

 several species of Rubus, which had survived from previous periods, met and settled 

 down together in Central Europe. On the coasts of Dalmatia and Greece, where 

 only a single species, viz., Ruhus ulrtiifolius, Schott ( = R. aiiioenus, Portenschlag), 

 established itself when the changes in question took place, there was no possibility 

 of any multiplication of forms. From that solitary species sprang descendants 

 which never changed; in other words, the specific marks of Ruhus ulrtiifolius 

 remained permanent in the above-mentioned parts of the Mediterranean floral area. 

 The idea of the old school of Botanists was that this particular species of Ruhus 

 had no inclination to evolve new species; or, to use the more erudite but still less 

 intelligible mode of expression, it was destitute of any tendency towards differentia- 

 tion. The proper explanation of the fact is, however, much simpler and more 

 natural. In the region where this particular species of Ruhus is settled, there is no 

 possibility of new species of that genus arising by means of inter-crossing. Perhaps 

 some future displacements of floras will bring Ruhus ulmifolius into proximity 

 with other species of Bramble, and in that case it is sure to take part once more in 

 the generation of new species. If, on the other hand, some event should cause 

 the extermination of the entire Bramble-flora in the adjacent districts, and Ruhus 

 ulmifolius should remain isolated, no new species will spring from it any more 

 than hitherto. A completely isolated species may continue to reproduce and 

 multiply sexually and asexually for centuries without exhibiting any alteration, 

 provided that the conditions of climate and soil are congenial to it, but it cannot 

 take any part in the production of new species. If at length destruction befall such 

 a species — a by no means impossible contingency, as one change of climate succeeds 

 another, and causes fresh displacements of the limits of plant distribution — the 

 event would connote the extinction of the whole genus of which this species was 

 the sole surviving representative. 



The result of comparative researches has been to show that the extinction of 

 single species frequently occurs, whereas such groups of species as Botanists designate 

 by the name of genera rarely die out. By far the greater proportion of the plants 

 whose remains have been preserved from former periods in a fossil condition 

 belong to genera which are represented by plants still living at the present day; 

 only many of the extant representatives difier specifically from those which existed 

 long ago. We conclude that the living types replace extinct ones and have entered 

 upon their parts in life. It is also worthy of note that the fossil remains are often 

 found at entirely different places from those inhabited by their nearest relatives in 

 modern times. 



