64 



natural selection, if disadvantageous to the species, but 

 which may be seized upon, and thus made a factor in the 

 evolution of a new form, if it be advantageous. 



The details of the facts of variation are only known in a 

 few classes of animals, and in these in a comparatively small 

 number of species ; but study shows that every organ and 

 part, even the quality and nature of the tissue forming each 

 organ, are subject to great and continued variation. The 

 close study of a single species of our British Lepidoptera in 

 the whole range of its geographical distribution, will show 

 that these variations are not the small and comparatively 

 unimportant differences that some opponents of the Dar- 

 winian theory have asserted them to be, but considerable and 

 marked in their extremes, with as a rule intergrading forms 

 between the extremes, sometimes filling up the gap by 

 almost imperceptible gradations ; whilst in others the grada- 

 tions appear to be less numerous, the extremes being dis- 

 tinctly marked and apparently very different in their general 

 facies. These, so far as I understand, are included with 

 sports or chance aberrations that occur but rarely in the life 

 of a species, as "discontinuous variations" by Bateson, yet 

 in their essence it appears to me they are distinctly different 

 both in their nature and essential characters. Probably I 

 shall have occasion to refer to these so-called " discontinuous 

 variations " later in this paper. 



The rapid multiplication of species and their enormous 

 fecundity are other important factors in the scheme of 

 organic evolution. The rapidity with which many species 

 multiply is, as a rule, far greater than is generally assumed, 

 and this is especially the case in what are termed the lower 

 forms of organic life. Their fecundity, too, or reproductive 

 power is equally amazing. It has been estimated that a 

 single puffball produces 600,000,000,000 of spores to a cubic 

 inch, and it is known that a single Wood Leopard Moth 

 {Zeuzera pyrina) lays on an average above looo eggs. Assum- 

 ing that the progeny of a single puffball lived and came to 

 maturity, it is evident that in a few years the whole earth 

 would be covered with puftballs ; and if the whole progeny 

 of a single Zeuzera pyrina lived, say for five or six genera- 

 tions, the sexes being equally divided, the females would 

 increase as i : 500 : 250,000 : 125,000,000 : 62,500,000,000 : 

 31,250,000,000,000, the sixth generation numbering in both 

 sexes 62,500,000,000,000 examples. In a few years the 

 whole of our timber trees would be destroyed by the amazing 

 numbers of Zeuzera developed. It is, however, well known 



