KEAPPEARANCE OF SPECIES. 183 



limited, and only in the comparatively very recent Tertiary epoch 

 do they acquire any marked significance. But, if the number of 

 survivors of any one epoch is very limited, it will naturally follow 

 that this number will be still further reduced if a question of two 

 or more epochs is involved. It was, indeed, for a long time main- 

 tained by geologists that no species of animal, no matter of what 

 form of organisation, could possibly have entered into the forma- 

 tion of three successive [epoch] faunas ; for instance, that no species 

 appearing in the Silurian could live comijletely through the Devo- 

 nian, and then continue into the Carboniferous. And this notion 

 is still largely entertained by the geologists and paleontologists of 

 the present day. However much of truth there may be in such a 

 doctrine, it must be confessed that its acceptance or rejection will 

 depend, at least in some part, upon the standpoint from which the 

 investigator views the nature of species. There is, perhaps, nothing 

 that has more taxed the ingenuity of the naturalist than to deter- 

 mine just exactly what a species is, what constitutes its absolute 

 boundaries or limitations. The amount of disagreement upon this 

 point is so very great, even among naturalists holding approximately 

 the same views on genesis or evolution, that one might fairly despair 

 of arriving at anything like a just solution of the problem. During 

 the period when the doctrine of the immutability of sj^ecies was a 

 common faith, there was, indeed, but little difficulty in the matter, 

 since every reasonably differing form was immediately constitut- 

 ed into a distinct species; and it is in this period that many, if 

 not most, of our existing paleontological notions have their roots. 

 Now, however, when the doctrine of descent by modification is 

 universally acknowledged as one of the great truths of nature, much 

 greater latitude is permitted to the definition of the word species, 

 and, in fact, what might at one time be considered as a good species 

 can, in the light of a newly-discovered chain of " intermediate " 

 forms, be readily degraded to the rank of a variety. Such has, in 

 truth, been the history of a great many so-called species. The 

 difficulty of determining specific longevity thus becomes apparent, 

 for who can state what will be the fate of forms that now stand 

 apparently far apart ? However divergent may be the views of 

 authors on the matter of relationship, it is practically certain that 

 numerous forms of life, exhibiting no distinctive characters of their 

 own, are constituted into distinct species for no other reason than 



