No. I.] VARIATIONS [IN LIMULUS POLYPHEMUS. gI 
the outset, and for that reason responded ina different way to the 
same environment. Again, if itis the environment alone that pro- 
duced the excessive growth manifested in the formation of two 
or three embryos out of one, we should expect that under these 
favorable conditions the new embryos would be large and 
vigorous, but as a matter of fact they are not, for no sooner 
are they once formed than they begin to degenerate, and 
finally all of them may be reduced to mere remnants. 
Hertwig’s criticism of Weismann’s explanation of polymor- 
phism in ants and bees, as well as of multiple embryos, is, it 
seems to me, a valid one. For, as he points out, we have no 
right to assume that the embryo is provided with several sets 
of germ plasm destined to develop, under proper stimuli, into 
new embryos or organs, unless the necessary stimuli are likely 
to occur in nature. And it might also be added that this con- 
dition could not have been fixed by natural selection, if its 
realization brought death with it. 
On the other hand, the assumption that Hertwig makes, that 
the stimulus of a new environment is alone sufficient to pro- 
duce new formative material, is not a necessary one. Indeed, 
the facts seem to me to point to an opposite conclusion. 
The factors producing forced growth of this kind, it seems 
fair to assume, are the reverse of those that cause median 
concrescence, because the sequence of events is nearly the 
reverse. That is, excessive tension causes organs to fuse and 
disappear along the median line. Is it not probable then that 
abnormal reduction of this tension would facilitate the forma- 
tion of new organs there, which will then reverse the tension 
conditions and cause the organs to disappear ? 
The formation of multiple embryos may be the result of 
some inherent defect in the ovum, and the stimulus of a modi- 
fied environment may exaggerate these defects, and increase 
the percentage of multiple embryos; but there is no evidence 
to show among the higher, segmented animals, at least, that 
double or triple embryos may be produced by the sole action 
of any definite environment. 
The fact that double and triple monsters appear under abnor- 
mal conditions, is not a valid argument against the mosaic 
