GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 313 



Chalk formation, though the species are distinct in each stage. 

 This fact alone, from its generality, seems to have shaken Pro- 

 fessor Pictet in his belief in the immutability of species. He who 

 is acquainted with the distribution of existing species over the 

 globe, will not attempt to account for the close resemblance of 

 distinct species in closely consecutive formations, by the physical 

 conditions of the ancient areas having remained nearly the same. 

 Let it be remembered that the forms of life, at least those in- 

 habiting the sea, have changed almost simultaneously throughout 

 the world, and therefore under the most different climates and 

 conditions. Consider the prodigious vicissitudes of climate during 

 the pleistocene period, which includes the whole glacial epoch, 

 and note how little the specific forms of the inhabitants of the 

 sea have been affected. 



On the theory of descent, the full meaning of the fossil remains 

 from closely consecutive formations being closely related, though 

 ranked as distinct species, is obvious. As the accumulation of each 

 formation has often been interrupted, and as long blank intervals 

 have intervened between successive formations, we ought not to 

 expect to find, as I attempted to show in the last chapter, in any 

 one or in any two formations, all the intermediate varieties be- 

 tween the species which appeared at the commencement and close 

 of these periods: but we ought to find after intervals, very long 

 as measured by years, but only moderately long as measured 

 geologically, closely allied forms, or, as they have been called 

 by some authors, representative species; and these assuredly we 

 do find. We find, in short, such evidence of the slow and scarcely 

 sensible mutations of specific forms, as we have the right to ex- 

 pect. 



ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED 

 WITH LIVING FORMS 



We have seen in the fourth chapter that the degree of differ- 

 entiation and specialization of the parts in organic beings, when 

 arrived at maturity, is the best standard, as yet suggested, of their 

 degree of perfection or highness. We have also seen, that, as the 

 specialization of parts is an advantage to each being, so natural 

 selection will tend to render the organization of each being more 

 specialized and perfect, and in this sense higher; not but that it 

 may leave many creatures with simple and unimproved structures 

 fitted for simple conditions of life, and in some cases will even 

 degrade or simplify the organization, yet leaving such degraded 

 beings better fitted for their new walks of life. In another and 



