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know that a few rabbits can stock a continent with their kind. May not, therefore, all the 
Nesokias be derived from an original pair or two which were in the first case the offspring of 
Gunomys ? 
The supposition that every species was derived from a single pair is so ancient as to appear 
ridiculous, but the scientist can only say that there is no evidence in favour of it. But is there 
no evidence in favour of it ? If it can be shown that there are in the world groups of animals 
of very limited numerical strength (less than a hundred) each member of which possesses certain 
special characters uniting it with its group fellows and separating it from all other animals, the 
demonstration will in my opinion afford evidence in favour of this supposition. I have shown 
elsewhere* that such groups not only exist, but are common, at least among the rats of India. 
In order to obtain knowledge of these groups it is necessary to examine animals in very large 
numbers and over wide areas. At the present day such groups are overlooked because it is the 
custom of naturalists to search for new species. A new species is in itself considered to be an 
interesting thing. A type specimen is chosen as a representative of the species and the 
supposition is thereby made that this type specimen is one of an unknown but large number of 
like animals. But the numerical strength of the "species" is never enquired into, it may not be 
large, it may be ten, fifty or a hundred. 
A new species may be confined to a single house or to the corner of a field or even to a 
single nest. Rare species are indeed plentiful. It may be asked — " What has all this to do with 
the question of fertility V I will endeavour to show the connection. A small group can only 
become a large one when production is in excess of elimination. A new group will grow up 
among an old group either because its new features are of life-saving value to it and tend to 
reduce elimination, or because the new group has a higher rate of production than the old one. 
Since the distinguishing marks of species do not as a rule appear to be of life-saving value to 
their possessors, I conclude that in many cases the new groups must grow in numbers simply 
because they have a higher rate of production, but this conclusion is untenable if we are to hold 
with Professor Pearson that differences of fertility are never inherited. 
VII. Remarks on Professor Lloyd's Note on Inheritance of Fertility. 
By KARL PEARSON, F.R.S. 
Professor Lloyd says that no new principle was deduced by him from his observations, and 
again that he "propounded no principle bearing on Evolution or any other subject." Either he 
has overlooked what he himself wrote, or else he must have very vague ideas of what does bear 
on Evolution. Yet he wrote p. 262 of his memoir : "The result obtained was quite unexpected 
[presumably therefore it was new]. The maximum fertility of rats (as measured by the number 
of young which they produce at a birth) is not one with the character which is the type 
as regards size. In other words, gigantic and dwarfed rats are just as fertile as common rats of 
average size." 
Thus Professor Lloyd himself says that as far as size was concerned there was no " maximum 
fertility." Professor Lloyd used this result to combat the principle that, if there were a 
maximum fertility, it must be associated with the modal value of the character, or the race 
could not be stable. How where there was in his opinion no maximum fertility observable 
in his experiments, he could use its non-existence to combat a statement of where it would 
occur, if it existed, I fail to understand. 
He further states that he laid no stress on this (B) absence of differential fertility in rats, 
and that he did lay stress on (A) the maximum fertility of rats not being associated with 
the type as far as size is concerned. As he had, according to his own interpretation of his 
* Records Indian Museum, Vol. nr. Pt. i. and Vol. v. Pt. u. 
