1884*] Theory of Evolution. 189 
75 per cent of the crows are thorough-bred carrions, perhaps 
5 per cent pure hoodies, and the remaining 20 per cent are 
hybrids of every stage between the two. He found pairs of 
such hybrid birds mated together, and producing eggs and 
young. This instance is the more valuable because it meets 
the cavil that though a hybrid may fecundate, or be fe- 
cundated by, an animal of the original pure strain, yet two 
hybrids cannot prove prolific together. 
It is on account of this cavil that I lay little weight upon 
the instance of the she-mule now in the gardens of the 
Society of Acclimatisation, of Paris. This animal has 
brought forth six foals, by zebras, by an ass, and by a stal- 
lion. 
The third proposition is therefore fully refuted. If there 
were between distinct species some absolute, primordial, 
Divinely-ordained boundary to prevent the intermixture of 
species, we might surely expedt that in every case copulation 
between animals of such different species would be abso- 
lutely null and void. In other words, in such a case the 
male element would produce no more effedt upon the germ- 
cell than would the contadt of a drop of water, or of blood, 
or of milk. But in place of this absolute adtion or nullity 
we find a series of effedts ranging from nullity upwards. In 
some cases there is doubtless no adtion at all. In others 
the germ-cell exhibits segmentation more or less normal, 
which, however, becomes prematurely arrested. In a third 
set of cases there is a produdtion of living young, and in a 
fourth these young are themselves capable of reproduction. 
These fadts are totally irreconcilable with the theory of an 
absolute boundary. In part they are already explained by 
very simple mechanical considerations ( see Dr. Pfliiger’s 
researches on the Hybridisation of Amphibians, “ Journal of 
Science,” 1884, p. 30). Thus in some cases the spermato- 
zoids, from their size and shape, are prevented from entering 
the ovum owing to the narrowness of the micropyle. All 
this, I repeat, is quite out of harmony with the teachings of 
the Old School. 
There is another consideration not to be overlooked : when 
a negative dogma is to be met by positive instances, and we 
find a notable amount of such instances after surveying only 
a very small portion of the field, we may not unfairly argue 
that on further inquiry these instances will be multiplied. 
On the other hand, they cannot be diminished. Hence we 
have everything to hope and nothing to fear from further 
research. We see, e.g., among birds, how many instances 
of the produdtion of hybrids have occurred among those 
