Absorption Effects in the Spiral Nebulae.

appearance. They are so similar, indeed, that it is very doubtful whether they would have been recognized, even as varieties, if cytological examination had not shown them to differ in respect to their chromosomes,6 and if they had not been bred in the laboratory to test their compatibility. Here is an apparently clear case of incipient species formation. It seems practically certain that these two varieties, or species, have had a common origin, and that their individuality at the present time is due mainly to their incompatibility. Either of them would, in our opinion, pass for a mutant race of the other; and it is not difficult to imagine that they might have become differentiated from one anther in some such way as that outlined above; i.e., by mutations that brought about incompatibility. With the incompatibility once established they are now free to diverge more and more until they become clearly differentiated from one another. 4. To recapitulate: The evidence from two cases of incompatibility between mutants in laboratory cultures, together with evidence from what appear to be mutant forms and incompatible varieties in nature, tends to remove one of the most serious objections to the mutation hypothesis, and lays emphasis upon the possible evolutionary importance of mutations involving incompatibility.