INTRODUCTION. LS 
toward the completion of the whole from the largest to the 
smallest forms. If we could visit the whole of the stations on 
the face of the earth and could by any means obtain a view of the 
animals feeding at the bottom of the seas and rivers, and could 
obtain specimens of every variety of the shells, we might be able to 
so place them that the whole of the progressive development would 
be seen to be perfect from beginning to end. Starting with a cer- 
tain hypothesis and working in every direction, the more the inde- 
pendent facts tend to agree, without being strained to suit the 
theory, the greater will be the probability of the hypothesis 
being right. Commenting on the specimens we have been enabled 
to collect, and placing them in the order in which they seem to fall, 
we have long lines of varying shells passing in their descending 
order from the large to the very small; we have other lines interrupted 
in various parts of the series indicating that certain forms would fill 
the gaps and 1ender the whole of that section perfect. These forms 
may or may not exist, and we are compelled to seek in other 
divisions of the series for analogous examples. Applying the facts, 
easily to be observed in the series of shells of common occurrence, 
to the gaps that are open in rarer lines of descent, we are 
enabled to judge pretty accurately of the whole plan laid down, 
and in this way advance much nearer to the ultimatum than might 
have been expected. The constant changes in the direction of the 
lines diverging from the main stems produce the effect on the 
imagination that the whole series are more or less connecting lines 
between the great descending branches. 
Whatever may be the result of future investigation with 
regard to variation, it must be borne in mind that the good figures 
and descriptions furnished by the conchologists who have gone 
before us have been the principal sources from which we have had 
to obtain our information. In the future it is more than probable 
that the photographer will furnish us with such accurate repre- 
sentations of the shells as will render the study of them compara- 
tively easy. We may then be able to have a front and back view 
of each variety, more particularly of those taken in a single locality 
and at one haul of the dredge, 
