1863.] 
219 
blance, and that the % abdominal appendages have a general resem¬ 
blance because they belong to that group. 
If the above hypothesis, viz. that species often originate from struc¬ 
tural variations in their % reproductive systems, be correct, we can gain 
some glimpses of light upon the extraordinary and anomalous pheno¬ 
mena of hybridism. We can see dimly why species of distinct families 
never cross j why distinct species of the same family do not commonly 
cross in a state of nature; why varieties generally but not always cross 
with facility; why species apparently closely allied should sometimes 
cross with difficulty or not at all, and others apparently wide apart in a 
natural system should sometimes cross with facility; why certain genera 
of plants should be fertilized more readily by the pollen of distinct spe¬ 
cies than by their own; and why the S of a given species should cross 
readily with the 9 of another species, and the % of that other species 
cross with extreme difficulty or not at all with the 9 of the first. The 
fact that, with the imperfect resources at our disposal, we cannot detect 
in very many cases such structural variations in % reproductive organs, 
does not prove their non-existence. No chemist has yet succeeded in 
detecting any peculiar substance in the air of malarial districts;* yet 
that there is such a thing as malarial matter in the air, it is almost 
impossible to disbelieve. 
With regard to the question whether H. basalts Hagen, be a mere 
geographical race of H. americana Fabr., as hinted by Selys and Ha¬ 
gen, I incline upon the above principles to the opinion that they are 
distinct species, if it were only on account of the difierent structure 
of their % abdominal appendages. It may be assumed, it is true, that 
intermediate grades between the two structures exist in the country 
intermediate between the habitats of the two supposed species, as has 
been shown by Hr. Hagen to be the case in Gromphina with the Euro¬ 
pean Onycliogomplius forcipatus and Cordulegaster annulatus ; but 
until this is proved with regard to some one Calopteryginous species, 
the assumption is a very unsafe one. A character which is notori¬ 
ously constant throughout a large group in specimens of any given 
species from the same locality, cannot, I think, without some collate¬ 
ral proof be reasonably assumed to vary in specimens of another species 
belonging to the same group from difi’erent localities. 
Consolations in Travel, by Sir Humphrey Davy, p. 116. 
