the Introduction of New Species. 193 



physical conditions of a district, even small in amount if rapid, 

 or even gradual if to a great amount, would be highly un- 

 favourable to the existence of individuals, might cause the 

 extinction of many species, and would probably be equally 

 unfavourable to the creation of new ones. In this too we may 

 find an analogy with the present state of our earth, for it has 

 been shown to be the violent extremes and rapid changes of 

 physical conditions, rather than the actual mean state in the 

 temperate and frigid zones, which renders them less prolific 

 than the tropical regions, as exemplified by the great distance 

 beyond the tropics to which tropical forms penetrate when the 

 climate is equable, and also by the richness in species and forms 

 of tropical mountain regions which principally diff"er from the 

 temperate zone in tlie unifoi-mity of their climate. However 

 this may be, it seems a fair assumption that during a period of 

 geological repose the new species which we know to have been 

 created would have ap])eared, that the creations would then 

 exceed in number the extinctions, and therefore the number of 

 species would increase. In a period of geological activity, on 

 the other baud, it seems probable that the extmctions might 

 exceed the creations, and the number of species consequently 

 diminish. That such effects did take place in connexion with 

 the causes to which we have imputed them, is shown in the case 

 of the Coal formation, the faults and contortions of which show 

 a period of great activity and violent convulsions, and it is in 

 the formation immediately succeeding this that the poverty of 

 forms of life is most apparent. We have then only to suppose 

 a long period of somewhat similar action during the vast un- 

 known interval at the termination of the Palseozoic period, and 

 then a decreasing violence or rapidity through the Secondary 

 period, to allow for the gradual repopulation of the earth with 

 varied forms, and the whole of the facts are explained. We 

 thus have a clue to the increase of the forms of life during 

 certain periods, and their decrease during others, without re- 

 course to any causes but those we know to have existed, and to 

 efiects fairly deducible from them. The precise manner in which 

 the geological changes of the early formations were eff'ected is so 

 extremely obscure, that when we can explain important facts by 

 a retardation at one time and an acceleration at another of a 

 process which we know from its nature and from observation to 

 have been unequal, — a cause so simple may surely be preferred 

 to one so obscure and hypothetical as polarity. 



I would also venture to suggest some reasons against the very 

 nature of the theory of Professor Forbes. Our knowledge of 

 the organic world during any geological epoch is necessarily 

 very imperfect. Looking at the vast numbers of species and 

 groups that have been discovered by geologists, this mav be 



