Notices of New Books. 7607 



species ; Mr. Darwin admits the fact as one of measure, accountable, 

 he says, from those organic forms classified as varieties having closer 

 affinity than those distinguished as species. The evidence he disputes, 

 he says, is full of anomalies and difficulties, and has a false amount of 

 weight given it from the fact of the reputed phenomenon having been 

 erroneously used to define what forms are to be called species and 

 what varieties. The production of fertile offspring from two distinct 

 forms, whether they be species or varieties, is exceedingly arbitrary : 

 in some genera individuals widely differing can be crossed to produce 

 fertile progeny ; in others the forms more closely resembling each 

 other produce perfectly sterile progeny. Tlie species or varieties of 

 some genera are more easy to get first crosses from than others; and 

 yet the progeny are less fertile than from those which are more 

 difficult to obtain as first crosses. Again : the ease with which first 

 crosses may be obtained between two species depends on which is 

 employed as the male and which the female ; or, in other words, two 

 species do not always produce reciprocal crosses with equal facility ; 

 e.g., the male of A and female of B may more readily breed and 

 produce fertile progeny than the male of B and female of A. There 

 are some species that are more easily fertilized by the pollen of 

 another and distinct species than by their own pollen. Looking at all 

 these anomalous facts, and the great difficulty with which they beset 

 the subject of hybridism, Mr. Darwin does not consider that the 

 balance of evidence in favour of the increased fertility of the offspring 

 of crossed varieties over those of crossed species is greater than can 

 be justly accounted for b}^ the closer organic resemblance between 

 varieties than between species. We know that there is some limit to 

 the diversity of organic forms that will produce fertile progeny, and 

 we are justified in expecting to find the measure of sterility propor- 

 tionate to the measure of difference. 



The last difficulty Mr. Darwin discusses (Chapter XI.) is the fact 

 of certain apparent exceptions to the very general law, so essential to 

 his theory, that the area inhabited by a species is continuous ; for 

 if the same species occupied isolated parts of the world, between 

 which migration was impracticable, we should be driven to the 

 inevitable alternative, either that they were derived from distinct 

 centres of creation, or " the improbable assumption that species iden- 

 tically the same had been derived, by the process of natural selection, 

 from parents specifically distinct." 



The most prominent examples of isolation are of alpine species 

 being common to the summits of mountain ranges widely separated 



