290 THE CANADIAN NATURALIST. [Vol. vil. 



Phyllopods, Crinoids, and Ceplialopods in the older Palaeozoic ; 

 of Land Snails, Millipedes, Insects, Fishes, Labyrinthodonts, 

 Acrogens and Gymnosperms in the middle and later Palaeozoic : 

 of Belemnites, Dinosaurs, Ornithosaurs and other Reptiles, and 

 of Marsupial Mammals and Dicotyledonous trees in the Mesozoic; 

 of Placental Mammals and Man in the Tertiary and modern. 

 When they shall have shewn the gradations by which these, out 

 of the many cases which may be cited, have been introduced, 

 and this without assuming an imperfection in the record incred- 

 ible in itself and destructive of its value as a history of the earth, 

 they may be in a position to rebuke us for our unbelief. 



But it may be asked : — Have we no positive doctrine as to 

 the introduction of species ? In answer I would say that it is 

 conceivable that the origin of species may be one of those 

 ultimate facts beyond which science by its own legitimate 

 methods cannot pass, and that all we can hope for is to know 

 something of the modes of action of the creative force and of 

 the modifications of which species when introduced are suscep- 

 tible. In any case it is by searching for these latter truths that 

 we may hope tsUocesst'uUy to approach the great mystery of the 

 the oriiiin of lile. It is With reference to thes^e truths also that 

 the discussion ot" modern theories of d3rivation liu^ been chiefly 

 valuable, and in so far as establishe I they will remain is sub- 

 stantial results after th( sa theories have been exploded. Among 

 such truths I may mention the tni owing : We have learned 

 that in a'eological time species ten 1 to arise in groups of like 

 forms, perhaps in many parts of the world at once ; so that 

 genera and families culminate rapidly, then become stationary 

 or slowly descend, and become restricted in number of species 

 and iu lanire. Wo liave learned that in like manner each 

 specific ty[je has capacities for the j roductiou of varietal and 

 race fcrms which aie usually exercised to the utmost in the 

 early stages of its existence, and then remain fxed or disappear 

 and re-appear as circumstances may arise, und finally the races 

 fall off one by one as it ap^roiches extinction. Many of these 

 races and varieties constitute conventional species as distinguished 

 from natural species, and in so far as they are concerned, de- 

 scent with modificition occurs, though under very complex 

 laws, and admitting of retrogression just as much as of advance. 

 We have also learned that in the progress of the earth's history 

 embryonic, generalised and composite types take precedence ia 



