AND OF HYBRID PIGEONS. 
51 
have no clew. Darwin has pointed out repeatedly the curious 
parallel between crossing and the change produced by physical 
conditions. Animals and plants removed from their natural 
conditions are extremely liable to have their reproductive sys¬ 
tems affected. Still he recognizes that sterility is incidental 
and not a necessary concomitant of hybridism. In some fertile 
hybrids, indeed, there seems to be an improvement on the 
parent form. Thus Vernon^ found two species of sea urchins, 
which when crossed showed greater fertility and produced 
larger, stronger larvae than in direct fertilization. 
SUGGESTIONS. 
The above conception of the chromatin does not necessarily 
imply that it is of exclusive importance as the basis for heredi¬ 
tary transmission. There seems to be no sufficient reason for 
not regarding the cytoplasm likewise as an important factor. 
Connected with the possibility of the qualitative differentiation 
of the chromatin there arises another possibility, namely, that 
only the individual and the more variable characteristics are 
conveyed by the chromatin, while back of these in the cyto¬ 
plasm is a general stock or plasma from which the more stable 
and constant form of an animal is derived. In other words, 
that there is a sort of double background of qualities to every 
organism which might perhaps be designated as general and 
individual. The cytoplasm provides the general substratum 
upon which all the later individual traits controlled by the 
chromatin are gradually built up. To illustrate in pigeons: 
what is recognized as the pigeon type or the general character¬ 
istics that mark a particular bird as a pigeon would constitute 
the first or general type, and those characteristics by which one 
recognizes different groups of pigeons or different individuals, 
would represent the second or individual type.. 
Such a conception, among other things, would simplify our 
interpretation of reversion to a distant ancestral form. With 
reference to the actual occurrence of such reversion it may be 
said that there are sufficient well authenticated cases on record 
to establish beyond a doubt its existence. Among others, 
1. Vernon, H. M: The Relation between the Hybrid and Parent Forms of 
Echinoid Larvae.—PhiJ. Trans. Series B. CXC, 189. 
