Dr. F. Miiller on a Hybrid Balanus. 411 



pairs of setge 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 

 natm-al course to explain the astonishing mixture of the cha- 

 racters of B. armatus and B. assimiUs 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 Balani 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. 

 assimiUs which had wandered into a tuft of Carijoa, and here, 

 deeply hidden, was surrounded only by B. a7^matus. If this 

 explanation be correct, our hybrids would be produced from 

 ova of B. assimiUs fertilized by semen of B. armatus. 



A further question raised by these hybrids is, why they 

 have received from B. assimiUs 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. assimiUs, and the broad smooth radii and 

 sexdentate labrum of B. armatus, differ less from the ordinary 

 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. 

 assimiUs. 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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