82 KEV. A. IRVING^ D.SC, ON EVOLUTIONARY LAW 



through which life-forms passed, from tlie fauna which broadly 

 characterises the paheozoic, to that which broadly characterises 

 the Mesozoic series of stratified rocks.* It is well to emphasize 

 the fact that there was no occurrence even at that stage of 

 wholesale or sudden exterminations, or of the sudden appearance 

 of new forms on a general scale. Yet in a general sense we can 

 diherentiate the life-forms of the one period from those of the 

 other. It is only in the Mesozoic age, when we may fairly 

 suppose that the composition of the atmosphere became pretty 

 nearly what it is at present, that tvarm-hiooded animals, which 

 require not only a 'plentiful supply of free oxygen, hut also the rapid 

 elimination of 00^ from their hlood,\ appear in the form of birds ; 

 while the same period of the earth's history was marked by the 

 appearance of "great sea-monsters" {Ichthyo-, Plesio-, and Plio- 

 s«2m^s), along with a prolific and abundant marine fauna including 

 bony fishes ; and phanerogamous j^lants seem to have gradually 

 attuned their mode of existence to the present constitution of the 

 atmos23here.+ Broadly, as the result of evolutionary change, 

 2)assu with changes of physical conditions in the environment,§ 

 we can recognise a gradual and progressive advance in the life- 

 forms which appear upon the stage of the world, over those which 

 prevailed in paheozoic times ; and without doing violence to the 

 narrative freely interpreted, on principles already assumed, we 

 may fairly connect all this with what is stated in verses 20-23 

 of Genesis i. 



When we pass on to the Tertiary age, we find tliat this again 

 presents its broad general characteristics, the most noteworthy 

 of which is the great development of the mammcdia^ the first 

 dawn of mammalian life having appeared rather late in the 

 Mesozoic ageJI though only to such an extent as to have been 

 quite subordinated to the other great classes of the vertebrata ; 

 and the tertiary mammals range in an unbroken series down 

 to the present day, as the ancestry of the mammalia now living 

 on the globe. 



* The present writer's work in this department of geology may be 

 found summarised in his paper, " Twenty years' work at the Younger 

 Eed Kocks," Geol. M(tg., August, 1894. 



t See iVat'or, vol. Lxxii, p. 355, for a remarkable lecture on Eespira- 

 tion by Dr. Stirling, at the lloyal Institution. 



To reach their full (leveloi)ment in Tertiary times. 



^ Chiefly — (i) lowering of temperature and diminution of salinity of 

 the ocean waters ; (ii) purification of the atmosphere from an over-dose 

 of CO,. 



II No one, 1 fancy, believes in the Mcrolestes i\ow, anymore than in the 

 Eozoon Canadense. 



