26 



should, out of the iBnumerable possible mates meet with one of 

 the opposite sex similarly circumstanced, or, at any rate, free 

 from disadvantage ; and that succeeding generations should be 

 equally fortunate until the variation should become permanent. 

 This " chance upon a chance " demonstrated to a mathematical 

 certainty that some controlling force, not hitherto recognized by 

 scientific theorists, must operate to control the perpetuation of 

 forms of vai-iation. He remarked that the usual explanation 

 given of the difficulty (viz., the enormous number of individual 

 cases from which the selection had to be made) was, when 

 properly understood, seen to be really the root of the difficulty 

 which requires explanation. 



He also pointed out the great difficulty involved in en- 

 deavouring by the existing theories of natural selection to account 

 for the perpetuation of periodical variation in the plumage 

 of the sarce species, as for example the assumption of white 

 plumage in snow-time or of more brilliant plumage in spring- 

 time, to be succeeded annually by a return to another state 

 when the snow has melted or the summer gone ; this difficulty 

 not being in the least diminished by the obvious advantages 

 accruing to the species by the variations in question. Also in 

 accounting for such well marked and persistent differences as are 

 found in the two forms of the Arctic Skua or the common 

 Guillemot mentioned in the former lecture. 



WEDNESDAY, MAY 9th, 1894. 



THE ORIGIN OF BIRDS, 



BY 



MISS AGNES CEAXE. 



Modern Science regards birds as having been remotely 

 evolved from ancient reptilian ancestors by gradual processes of 

 natural transformation during the lapse of incalculable ages of 

 geological time. Existing birds may be said to differ princi- 

 pally from living reptiles in the possession of a four-chambered 

 ■heart, warm blcod, and an external covering of feathers. Many 



