Dr. F. Miller on a Hybrid Balanus. 411 
pairs of sete on the posterior cirri), stand exactly in the middle 
between the two species. ) 
From all this, it seems to me to be the simplest and most 
natural course to explain the astonishing mixture of the cha- 
racters of B. armatus and B. assimilis which our animals 
show, by a true intermixture, and therefore to regard them as 
hybrids of the two species. 
But why, it will be asked, if this supposition be correct, are 
not hybrids of Balant remarkably abundant, if they occur at 
all? The different species so commonly dwell intermixed with 
each other, that three or more species may not unfrequently be 
found united in the same group. ‘To this I can only answer 
with suppositions. In order to obtain hybrids of plants, the 
stigma must be carefully protected from the pollen of the same 
species. If pollen of the same and of another species be placed 
upon the stigma at the same time, the latter remains inactive. 
In the same way, in animals, if the semen of the same and of 
another species be simultaneously in contact with the ovum, 
the latter may remain inactive. Now, wherever species of 
Balanus reside together in abundance, the ova will never miss 
the semen of their own species, and therefore no production of 
hybrids will take place. This can only occur when the ova 
of one animal come in contact only with the semen of a different 
species. Now this might easily be the case in an isolated B. 
assimilis which had wandered into a tuft of Carzjoa, and here, 
deeply hidden, was surrounded only by B. armatus. If this 
Be enation be correct, our hybrids-would be produced from 
ova of B. assimilis fertilized by semen of B. armatus. 
A further question raised by these hybrids is, why they 
have received from B. assimilis precisely the formation of the 
walls, opercular pieces, cirri, &c., and from B. armatus the 
precise structure of the radii, labrum, &c. It may be said 
that the merely transversely striated scuta and the weakly 
armed cirri of B. assimilis, and the broad smooth radii and 
_ sexdentate labrum of B. armatus, differ less from the ordina 
characters of the genus than the deeply pitted scuta and the 
_ strong teeth on the cirri of B. armatus, or the narrow radii 
clothed with membrane and the 22—28-toothed labrum of B. 
assimilis. "This applies also to the uniformity of the hairy 
covering of the opercular pieces. But by this means the 
matter of fact is only brought under a general point of view, 
and not explained. Out of this difficulty in this case, as 
usual, we can hardly escape without Darwin’s theory of the 
origin of species. But if we regard the species of a genus as 
descendants of a common primitive form, and at the same 
time, in accordance with the well-known experience of gar- 
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