90 ON VARIETIES, RACES, SUB-SPECIES, ANB SPECIES. 



2. MODIFICATIONS OF TWO ORGANIC FORMS CONSIBERED IN 

 A HYBRID ORIGINATING FROM TWO INDIVIDUALS OF DIF- 

 FERENT SPECIES. 



Having now considered the principal causes capable of modi- 

 fying the individuals of one and the same species, we have next 

 to complete our account of the causes which alter organized 

 bodies, and to examine tlie mutual influence of two individuals 

 differing in sex and species, when they give rise, by the process 

 called hybridizing, to a hybrid individual. 



Hybrids considered with respect to the definition of species and 

 to the importance of their study. 



We know that there are individuals of different sexes and 

 species which can generate what are called hybrids ; we know too 

 tliat animal hybrids are usually called mules. We must bear in 

 mind tliat the word species, as used by gardeners, includes 

 varieties, races, and sub-species ; and consequently what they 

 call a hybrid is often nothing more than a cross between two 

 varieties, two races, or two sub-species of one and the same 

 species ; tlie distinction between such crosses and true hybrids 

 must never be lost sight of, and we shall, to avoid all confusion, 

 hereafter denote such crosses by the term sub-hybrids. 



When we look at hybridizing in its proper sense, in an his- 

 torical point of view, we find tliat, like every other new subject, 

 it has been the object of the most unwarrantable generalisations, 

 attributable to the tendency there is in the mind of man to draw 

 inductions and form mere guesses, instead of confining himself 

 to positive conclusions deduced from facts ascertained by experi- 

 ment ; but if it be proved that hybridizing is only possible be- 

 tween allied species, that hybrids are generally, though not uni- 

 versally sterile, we must not run in the opposite direction and 

 undervalue the importance of a study of hybrids, the existence 

 and origin of which are incontestable. 



Although hybridizing is confined in narrow limits, when we 

 consider the number of its products, it is not on that account the 

 less valuable in making us acquainted with the influence of 

 ancestors on their descendants. 



Hybridizing is completely conformable to the ideas which we 

 have expressed in our definition of species, since it establishes 

 the following truths : — 



1. There are but few species among which it can take place, 

 and the organization of these species is exceedingly similar ; but 

 all species of which this may be said, do not, as a matter of 

 course, give rise to hybrids ; this explains why M. Sageret failed 

 in his endeavour to fecundate an apple tree by a pear tree. 



