ioo The Scottish Naturalist. 



age, the latest, only a fraction of the produce of any other. 

 Similarly, in the Neozoic period, the Tertiary epoch claims far 

 more than all the previous formations together, while the Trias 

 again, the earliest, produces, like the Permian, only its fractional 

 part. 



The grade of the organisms in the scale of animal perfection 

 is of no account. It is the amount of diversity of type alone 

 that is in question, and that is not affected by the considera- 

 tion how low or how high in the scale, were the forms which 

 the terrene conditions of the time were fitted to produce or 

 maintain. 



It may be pointed out that the. amount of genera originating 

 in Cretaceous times is a decline on that in Jurassic, so that the 

 theory on that point seems to break down. But, on considera- 

 tion, the exception may be felt rather to establish than abolish 

 the rule. Of all formations the chalk is the least known or 

 perhaps knowable in an adequately representative manner. It 

 has been nearly swept away — its rocks and its forms of life 

 together. The wonder is, as compared with the Jurassic, that 

 we have so many genera. An enormous lapse of time separates 

 all known Cretaceous rocks from the Tertiaries ; whilst the 

 evolution of organic forms takes in the interval a tremendous 

 leap — there being little community of generic forms, vertebrate 

 or invertebrate, uniting the two. The former fact explains the 

 latter. Probably for long cycles to come there will be no 

 resurrection of the intermediate deposits laid down at the cost 

 of the chalk. - If we could recover them, doubtless they would 

 disclose the line of continuity, physical and vital, and reveal 

 how much richer in generic forms the chalk really had been 

 than we now find it. There is no other way of explaining the 

 new and far different types of Tertiary ages, or the universal 

 unconformability of Tertiary rocks to the Cretaceous and the 

 denudation to which the latter have been subjected. 



But it will be suggested, this generalisation is now eighteen 

 years old — a very old age considering the rapid progress of 

 scientific research meanwhile. Facts bearing on the point 

 must have 'accumulated largely. Piclet must by this time be 

 much in need of a third edition. Suppose it were got up, 

 and the science brought down to the present day, would the 

 result read as before ? It is the progressive accumulation of 



