200 
PROFESSOR RARE PEARSON AXO ]\[TSS ALICE LEE, 
by a variety of other factors of evolution. These factors are so active that the 
influence is reduced to T2 of a child per generation if we take Case (i.) of Section (9), 
and, we have little doubt, would be practically insensible did we take all marriages 
without any limitation whatever. Reproductive selection must, therefore, be looked 
upon as always tending to increase the fertility of a race ; races are not only ever 
tending to increase, but tending to increase the rate at which they increase—a feature 
not recognised by Malthus, but which strengthens certain of his arguments. So 
soon, therefore, as environment, or‘other circumstance, relieves the pressure of 
op])osing factors, a race will not only increase in numbers, but also in fertility. It is 
this inherited character of fertility, and its constant tendency to change unless held 
in check by natural selection or other factor of evolution, which seems to us the 
source of the immense diversity in fertility to he observed not only in different species, 
l)ut in local races of the same species. 
III. On the Inherit (I,} tce of Fecundity in Thoroughbred Racehorses. By Karl 
Pearson, F.R.S., with the assistance of Leslie Bramley-Moore.'" 
(13.) The data ])rovided for the fertility of thoroughbred racehorses by the stud- 
hooJcs, are of a kind which cannot be hoped for except in the cases of pedigree animals 
kept for breeding purposes, and of specially-arranged experiments on insects, &c. 
We have a practically complete record of the stud-life of every brood-mare. The sire 
by whom she has been covered in eacli year is stated, and the result, barren, dead 
foal, living filly or colt, twins, &c., can be ascertained. It is also possible to find out 
whether the fi)al dies young, say as a yearling. By examining the whole series of 
stud-books the complete pedigree of any mare or sire can nearly always be found, 
and the correlation theoretically worked out for almost any degree of relationship. 
In starting an investigation of this kind on such a great mass of raw material, 
it is necessary to draw up certain rules for the extraction and arrangement of data. 
These rules must he prepared without any definite knowledge of the character of the 
material in bulk, for this can only be found after, perhaps, some 1000 cases have 
been extracted and worked out. Hence the rules originally adopted are often not 
such as an investigator would have arranged had he known beforehand the general 
character of the conclusions he would reach. But the statistician cannot, like the 
experimental physicist, modify without immense labour his methods and repeat his 
experiment. The collection of Ids data has frecpiently been far too laborious a task 
for repetition. His raw material lias been prepared in a certain manner; he may 
* Diii'ing the tlireo years in which this investigation has Ijeen in progress, a considerable nninher of 
friends have given me substantial aid in the arithnictioal work, or in the ])reparation of the 6,000 
jiedigree cards on v.diich the results are based. iJIr. RKAMr.EY-]\rooRi3 has latterly been iny chief helper, 
but T am also much indebted to Miss AriCb; Lee and i\rr. G. U. Yule. Afiss Margaret Shaen and Miss 
Li.RA Eckenstein have also contributed to the labour of extracting the raw data from the stud-book.s. 
