423 SOSS Om EVOLUTION. 



The simpler Protozoans seem to consist of a single cell or of an 

 ao-o-reeation of simple cells, without any of that differentiation of 

 tissue wliich characterizes the higher Orders of animals, and though 

 some' of the higher Protozoans secrete silica, and others, carbonate 

 of calcium, yet not so as to form a tissue; so that these first of 

 Animals exhibit a protoplasmic basis for animal life such as exists 

 in the earliest stage of each individual animal (or community of 

 individuals in the case of such animals as are produced m numbers 

 from the product of a single egg) in the earliest stage of the egg. 

 As we rise in the scale of animal life, or, as we follow the succes- 

 sion of fossils upwards in the formations, or as we follow the 

 progress of differentiation in the egg (in the higher animals) we 

 find in each a corresponding localization of function, and suitable 

 specialization of tissue. It is true that in comparing the first two 

 it is necessary to allow for the greater specialization of the later 

 animals as compared with the earlier of approximately the same 

 grade, while we labor under the serious disadvantage of having so 

 ]ittle of the tissue of earlier animals well preserved or in any way 

 directly indicated ; and that in comparing the latter with the others 

 it is necessary to allow for the fact that in the egg neither armor 

 for defence nor weapons for attack are needed, nor is there any 

 functional organization for reproduction. But allowing for all 

 these the analogy seems perfect. 



Dana estimates the comparative duration of the Post Tertiary, 

 Tertiary, Mesozoic, and Palaeozoic Periods, as approximately 

 expressed by the numbers 1, 2, 4, 14, respectively. Sir Wm. 

 Logan's estimate of the thickness of the Cambrian and Laurentian 

 rocks of Canada, be taken as the maximum thickness of these, it is 

 probable that they represent a period equal in duration to the 

 Palaeozoic Period. So that the various fossiliferous rocks may be 

 estimated to have occupied a Period equal to thirty-five times that 

 of the Post Tertiary, which was probably not less than half a 

 million years, so that for the accumulation of the twenty-one miles 

 in the thickness of the various Zoic Formations, it will be safe 

 to estimate the minimum duration at fifteen millions of years, 

 though it may have greatly exceeded even this immense Period. 

 But was the Epoch of Eozoon^ indeed that of the *' dawn " of life? 



