SPECIES IN THE MAKING 115 



intermediate forms inhabiting an intermediate area. Thus 

 the one species becomes two, and these may again break 

 up, and, having become thus disconnected and stabilized, 

 they may spread over one another's territory — fly side by 

 side and yet remain distinct forms which do not pair 

 together — although originally they were varieties spread- 

 ing from a common centre, where the ancestral species 

 lived and multiplied. 



Other similar gradational series of an interesting 

 character have been noticed in the case of fresh-water 

 fossil snail-shells. In the layers of clay and marl exposed 

 by digging a railway cutting or a pit we may find that the 

 successive layers represent a continuous deposit of loo.ocK) 

 years or more, and we find sometimes that a form of 

 snail-shell (not a species living to-day) occurs in the 

 lowest stratum very different from that occurring in the 

 highest stratum — the lowest being short and spherical, 

 the highest elongated and of differing texture. In 

 the intermediate layers, each 6 or 12 ins. thick and 

 occupying perhaps altogether 30 ft. of vertical thick- 

 ness, we find a graduated series of snail-shells leading 

 almost imperceptibly from the oldest lowest form to the 

 latest uppermost form. Such cases are known. But it 

 is an exceptional thing to find these graduated series 

 either spread over an area of the earth's surface, or 

 following one another in successive strata. When they 

 came into existence they were rapidly superseded and 

 destroyed as a rule, and have left only one or two widely- 

 separated examples of the intermediate forms. This we 

 should naturally expect by analogy from what we know 

 of the successive traces of human manufactures in the 

 deposits on the site of some of the great cities of the 

 ancient world which have been carefully excavated layer 

 by layer. But still we have the important fact that here 

 and there such gradational series have been found, and 



