CHAPTER XII 
SPECIES HYBRIDIZATION 
In the preceding chapters an attempt has been made to show how 
character differences in a large number of plants and animals may be 
interpreted on the basis of differences in the unit factors which are dis- 
tributed to the germ cells during gametogenesis. The character differ- 
ences, however, which were analyzed, although often seemingly complex, 
were really rather simple, for rarely were more than four or five factor 
differences taken into account. In a few species of plants and animals 
the number of factors which have been investigated is considerable, but 
when compared with the number of factors which must constitute the 
entire hereditary material of a species it is an insignificant fraction of the 
total. The analyses which have been presented, therefore, are for forms 
which possess an enormous number of factors in common. The differ- 
ences which they display are mostly unessential alterations in scattered 
loci in these systems. 
With the taxonomic question as to what constitutes a species differ- 
ence, we are not greatly concerned. It is clearly apparent that species 
as they have been named represent widely divergent differences with 
respect to the extent of separation from related species. It must be 
clear to the geneticist, therefore, that specific difference is a variable 
thing, sometimes meaning one thing, sometimes another. If we look 
at the question from the standpoint of the number of factors involved, 
we see clearly that races of plants and animals may differ in one or many 
genetic factors. Just where the line should be drawn which distinguishes 
varieties, forms, species, etc., would therefore appear to be almost wholly 
an arbitrary matter, usually to be decided from considerations of con- 
venience. Whatever it is, however, the distinction cannot well be viewed 
from the genetic standpoint, for ordinarily the systematist works with 
plants and animals which have not been investigated in such a fashion, 
and, in the case of the more widely separated forms, with those which 
cannot be so investigated. 
A genetic investigation of the difference between two species depends 
upon the possibility of crossing the species in question, and further upon 
the possibility of securing offspring from the progeny of such a cross. 
Not infrequently this latter condition is not fulfilled, for it often follows 
as a result of species hybridization that the individuals thus produced, 
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