TIME AND LIFE 137 



9 



and placoid fishes, having persisted from the palaeozoic 

 epoch to the present time without a greater amount of 

 deviation from the normal standard than that which is 

 seen within the limits of the group as it now exists. Even 

 among the Reptilia the class which exhibits the largest 

 proportion of entirely extinct forms of any one type, 

 that of the Crocodilia, has persisted from at least the com- 

 mencement of the Mesozoic epoch up to the present time 

 with so much constancy, that the amount of change which 

 it exhibits may fairly, in relation to the time which has 

 elapsed, be called insignificant. And the imperfect know- 

 ledge we have of the ancient mammalian population of 

 our earth leads to the belief that certain of its types, such 

 as that of the Marsupialia, have persisted with corre- 

 spondingly little change through a similar range of time. 



Thus it would appear to be demonstrable, that, not- 

 withstanding the great change which is exhibited by the 

 animal population of the world as a whole, certain types 

 have persisted comparatively without alteration, and the 

 question arises, What bearing have such facts as these on 

 our notions of the history of life through geological time ? 

 The answer to this question would seem to depend on the 

 view we take respecting the origin of species in general. 

 If we assume that every species of animal and of plant 

 was formed by a distinct act of creative power, and if the 

 species which have incessantly succeeded one another 

 were placed upon the globe by these separate acts, then 

 the existence of persistent types is simply an unintelligible 

 irregularity. Such assumption, however, is as unsupported 

 by tradition or by Revelation as it is opposed by the 

 analogy of the rest of the operations of nature ; and those 

 who imagine that, by adopting any such hypothesis, they 

 are strengthening the hands of the advocates of the letter 

 of the Mosaic account, are simply mistaken. If, on the 

 other hand, we adopt that hypothesis to which alone the 

 study of physiology lends any support that hypothesis 

 which, having struggled beyond the reach of those fatal 

 supporters, the Telliameds and Vestigiarians, who so 

 nearly caused its suffocation by wind in early infancy, 

 is now winning at least the provisional assent of all the 

 best thinkers of the day the hypothesis that the forms 

 or species of living beings, as we know them, have been 

 produced by the gradual modification of pre-existing 

 species then the existence of persistent types seems to 



