194 CHAPTERS ON EVOLUTION. 



that admittedly the panoramic views of evolution we are about to 

 discuss frequently present breaks and gaps in their succession. The 

 expanding canvas of life here and there exhibits a blank surface, due 

 to the part erasure of the picture which, we believe, formerly existed 

 thereon. 



There exists a second principle in nature and evolution, of equal 

 importance to heredity or that in virtue of which the likeness of the 

 parent or ancestors is transmitted to the offspring or descendants. 

 This second principle is that of " modification " by adaptation to sur- 

 rounding or varying conditions. The living being is a plastic unit, 

 capable of being affected and impressed in various, and often undeter- 

 mined fashions, by the forces of the world in which it lives. Such external 

 conditions heat and cold, food, habitat, and a host of other circum- 

 stances influence its development in the present, as unquestionably 

 in the past they have modified the history of its race. In truth, the 

 germ-idea of evolution is that of progressive change and alteration 

 induced by the great factors internal or innate, hereditary and vital 

 forces, and the external or outside circumstances of life. To the 

 operation and influence, then, of surroundings, acting variously upon 

 different natures and organisms, we rightly ascribe the deletion of 

 stages we would naturally expect to meet with in that recapitulation 

 of the animal evolution exhibited in its development. As the geo- 

 logical record, through its imperfections due to the metamorphism 

 and destruction of fossil-bearing rocks causes grievous gaps in 

 the history of past life on the earth, so the history and development of 

 the life of to-day shows its blanks and imperfections likewise these 

 blanks caused chiefly, we believe, by the varying outward conditions 

 under which the development of the race was carried out. Thus, if 

 the main outlines of the development of the frog-race be plainly 

 delineated, the pictures likewise may exhibit here but the dimmest 

 possible contour, and may there show a blank . The original fish-ancestor 

 of the race must be sought for amid the fossils possibly it may never 

 come to light at all. The successive stages whereby the tailed newt 

 became the frog, are barely outlined in the animal world of to-day, 

 and are here and there wanting altogether. But the finger-posts exist 

 nevertheless, and they guide our mental way satisfactorily enough, so 

 long as we trust to their indications. Even though we have to wade 

 through the high tides of difficulty and dimness of knowledge which 

 obscure the intervening ground, we may walk with confidence in that 

 sober path which is founded upon the reason that is attainable. 



As Huxley pertinently remarks in a recent manual of zoological 

 instruction : " In practice, however, the reconstruction of the 

 pedigree of a group from the developmental history of its existing 

 members is fraught with difficulties. It is highly probable that the 

 series of developmental stages of the individual organism never pre- 



