EXTIXCTIOX OF SPECIES. 197 



and if this could have happened once, why may it not have occurred 

 again ? Or might it be assumed that this, a primary oscillation, 

 first marked out the existing boundaries of land and sea ? The 

 problem, in whichever light it may be viewed, is beset with in- 

 numerable difficulties, and, it must be confessed, lies beyond the 

 probability of a near solution. The evidence appears strong, how- 

 ever, for concluding that the Archaean rocks, so recognised — i. e., 

 those of the Laurentian and Huronian series — are by no means the 

 immediate predecessors of the Cambrian series. These may still 

 be found at some places underlying the last, or they may forever 

 remain hidden from view" beneath the aqueous deep. 



Extinction. — It has very generally been remarked that the ex- 

 tinction of species, or groups of species, appears to have been a 

 much more gradual process than their introduction. This is, doubt- 

 less, in great part true, and agrees well with the theory of natural 

 selection. The formation of a new species usually implies favour- 

 able conditions for the development of that species, and it is, 

 therefore, not surprising that when once formed the species should 

 spread very rapidly. When, however, through certain causes — 

 the alteration of the physical properties of the surroundings or 

 inferiority in the general struggle for existence — the conditions 

 for existence are no longer as favourable as they were before, we 

 should naturally expect to meet with a decline in the development 

 of that species, and its possible ultimate extinction. But unless 

 the change in the conditions of life were very abrupt, we should 

 nowhere look for immediate or sudden extermination. Every one 

 is familiar with what prodigious rapidity certain weeds, as the wild 

 carrot, for example, have spread in regions into which they had 

 but recently been introduced, but how very much slower has been 

 the extermination of the species of native plants which they may 

 have supplanted. The English sparrow has developed with sur- 

 prising rapidity in the Eastern United States, and, although since 

 the period of its introduction scarcely twenty years have elapsed, it 

 has so far multiplied and become master of the newly acquired 

 situation as to have practically appropriated for itself a large por- 

 tion of the domain formerly occupied by the native birds of the 

 same family, and to the exclusion of those birds. These, if they 

 eventually prove weaker in the race, may in course of time com- 

 pletely disappear, but, before that period will be reached, will 



