678 
ASTRONOMY: H. D. CURTIS 
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 chromo- 
somes,^ and if they had not been bred in the laboratory to test their com- 
patibility. Here is an apparently clear case of incipient species forma- 
tion. 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 an- 
ther in some such way as that outlined above; i.e., by mutations that 
brought about incompatibility. With the incompatibility once estab- 
lished 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. 
1 In each case the two respective mutants appeared in pedigreed laboratory cultures, 
leaving no doubt about their being typical 'mutants.' 
2 For additional data on the mutants in D. virilis see Metz, C. W., Genetics, Princeton. 
1916 , 1 (591-607) ; for data on those in D. melanogaster see Morgan and Bridges, Washington, 
Carn^egie Inst., Pub., No. 237, 1916. 
3 Morgan and Bridges, op. cit. 1. 
^ Bridges, C. B., Genetics, Princeton, 2 (445-465). 
^ Sturtevant, A. H., Amer. Nat., Lancaster, Pa., 49, 1915 (190-192). Other cases of a simi- 
lar sort could be added; this one is used because it is taken directly from a Drosophila. 
6 Metz, C. W., Amer. Nat., Lancaster, Pa. 50, 1916, (597). 
ABSORPTION EFFECTS IN THE SPIRAL NEBULAE 
By Heber D. Curtis 
LICK OBSERVATORY. UNIVERSITYIOF CALIFORNIA 
Communicated by W. W. Campbell. October 18. 1917 
A study of the negatives of spiral nebulae obtained with the Crossley 
Reflector has shown that the phenomenon of dark lanes caused by occult- 
ing or absorbing matter is much more frequent than had previously been 
supposed. A paper of considerable length on this subject, in which the 
evidence is supplied chiefly by half-tone illustrations of seventy-seven 
spirals, will be pubKshed soon by the Lick Observatory. An abstract 
of that paper follows. 
