386 
culation is a vital stage in the proper progressive development of the 
embryo and is followed in normal embryos by a period of rapid growth, the 
question still remains, why does the circulatory system fail to develop 
properly? Why do we have so many embryos stop their development before 
the period of heart formation, and why do we have so commonly failures 
to emerge from egg, or die shortly after, when in the latter the circulation, 
at least to all appearances, has been normal? If we consider the experi- 
ments tabulated in Table 9 from the view point of the correlation between 
the degree of development and the relationship of the species combined, 
we see at once that only those species that belong to the same genus, or to 
very closely related genera, will produce hybrids that develop to the point 
of hatching. Even within this group a difference in this respect can be 
observed between species very closely related, and species more distantly 
related. ‘Thus Fundulus heteroclitus combined in either direction with 
Fundulus diaphanus will produce a large proportion of free swimming em- 
bryos. These two species, although the former is a marine and the latter 
fresh water, are structurally very closely allied. Fundulus majalis is much 
less closely related to Fundulus heteroclitus, although belonging to the 
same genus. When the latter is taken as the female a large proportion of 
vigorous fry are obtained. ‘The reciprocal has never yielded me embryos 
that would emerge from the egg, although, with the exception of the yolk 
bag, normal in appearance. Then the species used belong to separate 
genera the proportion of embryos that emerge normally is, as a rule, much 
smaller than in the preceding condition. 
All species that are more removed from each other than closely related 
genera, fail to produce hybrid embryos that will complete development to 
the point of hatching. Among this latter group of hybrids the stage to 
which development is carried varies considerably in the different com- 
binations. This, too, can be roughly correlated with the relationship of the 
species combined, so that two species belonging to distantly related orders 
like Fundulus heteroclitus x Tautogolabrus adspersus give rise to hybrids 
that can net go much beyond the closure of the blastopore, while if the same 
form is crossed with its nearer relative, Menidia notata, development pro- 
ceeds very much further although stops far from the point of hatching. 
This will be further taken up below. 
We produce, then, among fishes a series of hybrids that range in 
success from those in which none of the embryos develop very much be- 
