Hyatt.] 86 [Aprils, 
It is obvious tliat there is no struggle for existence in the kera- 
tose sponges of tlie Mediterranean as compared with those from 
Bermuda between these and those of the West Indies, nor yet 
between the species or zoons of tliese within these separate local- 
ities. The common characters that distinguish all the sjjecies of 
Keratosa in eacli basin are the direct product of the action of the 
surroundings, and the entergogenic reactions of the poriferous 
organism has made it suitable to its surroundings, not because 
nature lias selected some of its structures, but because it was con- 
sti-ained by these factors to grow in certain ways within each of 
these basins. Their characteristics in this case are jsrobably 
acquired in an analogous way to those of a drop of rain or of 
molten lead which becomes round in falling through the atmos- 
phere. If these drops possessed :in initial power of resistance and 
peculiar forms, and were only gradually modified as they fell into 
drops, the similar forms and the similar drops would be no more 
due to selection in one case than in the other. 
The only known cause of modification as demonstrated by the 
suitability of variations and existing characteristics, and by the 
more direct demonstration of experimentation, is of course the 
physical forces of the surroundings. These certainly have the 
power to originate modifications, either through their assumed 
direct action upon the growth of the parts, or through their power 
to excite entergogenic reactions and modifications in organisms. 
It is cei'tainly not a very acute analysis of the facts which 
attributes to external causes exclusive power in producing modifi- 
cations in m;my cases, as is now largely done by experimental 
zoologists. For example : Brauer and the author have both 
pointed out this defect in the accepted explanations of the famous 
Schmankewitscli experiments upon Artemia, and the same may 
be said of the explanations of all experimenters who do not take 
into account the reactions of the organisms themselves. 
Tlie physical forces of the surroundings must act through the 
medium of internal movements, and this is shown clearly in the 
nature of modifications produced whicli are extra growths or 
substitutions of characteristics due to changes of functions, etc., 
or the partial or absolute obliteration of these due to the failure 
of genetic force to repeat characteristics in the presence of op])os- 
ing influences and superimposed characteristics as in extreme 
cases of tachygenesis and cenogenesis. 
