Page - 201 - in The Origin of Species

DIFFICULTIES OF THE THEORY 201 possible in the other; and fundamental differences of struc- ture in the visual organs of two groups might have been an- ticipated, in accordance with this view of their manner of formation. As two men have sometimes independently hit on the same invention, so in the several foregoing cases it appears that natural selection, working for the good of each being, and taking advantage of all favourable variations, has produced similar organs, as far as function is concerned, in distinct organic beings, which owe none of their structure in common to inheritance from a common progenitor. Fritz jMiiller, in order to test the conclusions arrived at in this volume, has followed out with much care a nearly similar line of argument. Several families of crustaceans include a few species, possessing an air-breathing apparatus and fitted to live out of the water. In two of these families, which were more especially examined by Miiller, and which are nearly related to each other, the species agree most closely in all important characters ; namely in their sense organs, circulat- ing system, in the position of the tufts of hair within their complex stomachs, and lastly in the whole structure of the water-breathing branchiae, even to the microscopical hooks by which they are cleansed. Hence it might have been expected that in the few species belonging to both families which live on the land, the equally-important air-breathing apparatus would have been the same ; for why should this one apparatus, given for the same purpose, have been made to differ, whilst all the other important organs were closely similar or rather identical. Fritz Miiller argues that this close similarity in so many points of structure must, in accordance with the views ad- vanced by me, be accounted for by inheritance from a com- mon progenitor. But as that vast majority of the species in the above two families, as well as most other crustaceans, are aquatic in their habits, it is improbable in the highest degree, that their common progenitor should have been adapted for breathing air. Miiller was thus led carefully to examine the apparatus in the air-breathing species; and he found it to differ in each in several important points, as in the position of the orifices, in the manner in which they are opened and closed, and in some accessory details. Now such