412 Dr. F. Miiller on a Hybrid Balanus. 



deners, regard their various peculiarities as so much better 

 fixed, or so much less variable, the earlier they were acquired, 

 the longer they have been inherited unchanged, it becomes 

 intelligible that, above all, the characters proper to the primi- 

 tive form persist, and that consequently, in the crossing of two 

 species, these are more readily transferred to the hybrid than 

 later-acquired peculiarities of the father or mother. 



From this point of view I think we shall be able to explain 

 many peculiarities of hybrids and, vice versd, perhaps in 

 many cases to trace from the form of the hybrids to the 

 primitive form of the genus, — the latter, of course, only with 

 the greatest care ; for the mere fact that the hybrids produced 

 by males of one species with females of another do not agree 

 with those produced by males of the second species with fe- 

 males of the first, furnishes a proof that other circumstances 

 aid in determining the form of the hybrids. 



EXPLANATION OF PLATE XX. 



Fig. 1. Specimens of Balanus armatus, seated upon Carijoa : c, carina ; 



r, rostrum. 

 'Fig. 2. Scutum of B. armatus, seen from without, with particularly large 



pits and distant striae of gi-owth. 

 Fig. 3. Another (remarkably broad) scutum, from within. 

 Fig. 4. Hybrid of Balaams armatus and B. imju'ovisus. 

 Fig. 5. Scutum of B. imjM-ovisiis, var. assimilis, from without. 

 Fig. 6. The same scutum, from within. 

 Fig. 7. Outline of the mouth in B. armatus. 

 Fig. 8. Outline of the mouth in a hybrid of B. ar7natus and B. impro- 



visus. 

 Fig. 9. Outline of the mouth in B. improvisus, var. assijnilis. 

 Fig. 10. Labrum of B. armatus. 

 Figs. 11-13. Labrum in three different individuals of B. improvisus, var. 



assijnilis. 

 Fig. 14. Setse from the carinal side of the tergum in B. armattis. 

 Fig. 15. Setae from the scutal side of the same piece. 

 Fig. 16. Setae from the scutum in B. armatus. 

 Fig. 17. Setae from the tergum in B. imp7-ovistis, var. assimilis. 

 Fig. 18. Setae from the striae of growth on the tergum of B. improvisus, 



var. assimilis. 

 Fig. 19. Anterior ramus of the third pair of cirri of B. armatus, from 



vrithin, wherefore only the teeth which project beyond the 



margin are visible. 

 Fig. 20. The same ramus from the hybrid, from without. 

 Fig. 21. Ninth joint of the outer ramus of the third pair of feet in a large 



B. annatus, from without. 

 Fig. 22. Tenth joint of the outer ramus of the fourth pair of feet in B. 



armatus, from without. 

 Fig. 23. Seventh joint of the outer ramus of the third pair of feet in the 



hybrid, from without. 

 Fig. 24. The same joint of a B. impruvisus, var. assimilis. from without. 



