INVESTIGATION VERSUS PROPAGANDISM ' 335 



northward advance of pathogenic microbes, assignable to the rise 

 of temperature as an interglacial epoch comes on, be quite as 

 plausible a cause of extinction as the southward advance of a 

 cold climate as a glacial epoch comes on ? In this particular 

 case may not the advance of the warmth of the early Recent 

 Period have thus been indirectly one of the effective causes of that 

 rather remarkable rate of extinction which seems to be recorded 

 in the six feet of the creek deposits at Vero ? 



(2) The second line of approach has not been sharply distin- 

 guished from the first, nor has it been brought into equal use as 

 a means of discrimination, as the discussion of the Vero case 

 very pointedly shows. Has not the introduction of species, how- 

 ever, resources of discrimination equal to those of extinction, if 

 not indeed greater? In the use of extinctions the inquirer can 

 only start with the imperfect knowledge of earlier faunas and 

 floras. He can only trace these forward to disappearance by 

 means of the inferior record of those earlier times. On the other 

 hand, in the use of the introductions of species, it is possible, by 

 reversing the time-order, to start with the fuller knowledge of the 

 present faunas and floras and trace these backward by the use of 

 the more complete record of recent times. There may thus be 

 brought into use a large amount of data relative to the climatic 

 adaptations of existing species, varieties, and races, and hence by 

 inference, knowledge of the climatic conditions that obtained when 

 these were introduced. By starting with the richer resources of 

 the biologic knowledge of the present and the fuller knowledge of 

 present conditions, there is good reason to hope that before the 

 preceding records grow too obscure, the climatic effects of the 

 last ice invasion even upon southern faunas and floras may be 

 detected, and so the southern fossil records rendered directly 

 comparable to the glacial record. 



Now it is highly pertinent to the question at issue to observe 

 that the extinctions and introductions of biotic species, varieties, 

 and races during the last century have been phenomenal in kind 

 and degree. Within the memory of some of us that are older 

 there have been profound changes in the general aspect of the 

 regional faunas and the floras in North America more radical, 



