ZOOLOGY AND BOTANY, MICROSCOPY, ETC 33 



Race-crossing and Sex Ratio*-- Maud de Witt Pearl and 

 Raymond Pearl discuss the widespread belief that hybrids show an 

 excessive proportion of males, or, more generally, that the relative 

 proportions of the sexes may be influenced by the method of breeding 

 practised. Statistics of over 200,000 human births, extending over a, 

 period of ten years in the City of Buenos Ayres. show that the pro- 

 portion of males to females is significantly greater when the parents are 

 of different racial stocks than when they are of the same stock. The 

 preponderance of males in the offspring of cross-matings does nol 

 appear to be capable of explanation as the result of environmental or 

 demographic influences. 



Geminate Species.f — David Starr Jordan gives instances of what he 

 calls geminate species, which represent one another on opposite sides of 

 some form of barrier. Good instances are found among the fishes on 

 the two sides of the isthmus of Panama. Geminate species differ in 

 minor regards, characters which we may safely suppose to be of later 

 origin than the ordinary specific characters. " While segregation or 

 isolation is not a force, and, perhaps, not strictly a cause in species 

 formation, it is a factor which apparently can never be absent, if the 

 species retains its independent existence." 



Evolution without Isolation. £ — O. F. Cook does not think that 

 isolation (or selection) causes evolution, or helps it along. The separa- 

 tion of a species into two or more parts allows the parts to become 

 different, but there is every reason to believe that evolutionary changes 

 of the same kind would take place if the species were not divided. 

 Isolation is the shears that splits the species, not the loom that weaves 

 it. The weaving is done when the fabric is broad. The larger and 

 more diversified species make the truly constructive evolutionary 

 progress. The evolution of a species is in no way dependent upon its 

 being split into smaller groups, but is more likely to be hindered 

 by narrow subdivisions. If the groups are too small they degenerate 

 and become extinct, instead of continuing their evolution. Isolation, 

 though making more species, impedes evolution. 



b. Histology. 



Leucocytes in Invertebrates.§ — Max Kollmann has made an 

 elaborate study of the leucocytes in representative types of Inverte- 

 brates. He discusses the various kinds, and in particular the develop- 

 ment and role of granulated leucocytes. These accumulate reserve 

 substances, and increase or decrease in number as the state of nutrition 

 varies. The lymphoid organs found in various groups— molluscs, 

 arthropods, annelids, echinoderms— resemble similar organs in Verte- 

 brates, consisting essentially of stroma (usually cellular) with free 

 lymphoid cells in its meshes. 



* Biol. Bull., xv. (1908) pp. 194-205. 



t Amer. Nat., xlii. (1908) pp. 73-80. X Tom. cit., pp. 727-31. 



§ Ann. Sci. Nat. (Zool.) viii. (1908) pp. 1-240 (2 pis., 25 figs.). 



Feb. 17 th, 1909 D 



