ACQUIRED CHARACTERS IN ANIMAL BREEDING 49] 
for all the increase in size of Shetland ponies from generation to generation 
in the corn belt. To our knowledge it has not been determined accurately 
how great such an effect may be, nor how long it may persist. This is a 
point of some practical interest, but as to its relation to the inheritance 
of acquired characters, it is well to emphasize the fact that, as Thomson 
states, ‘‘experiments on increased size of parts are more decisive than 
those which refer only to the size of the whole.” 
An experiment designed to supply this need of data on change in 
size of particular body parts was conducted by Sumner, who subjected 
white mice to extreme temperature differences from the time of birth 
until 5 days before the females gave birth to their young. It was 
found that the offspring of warm-room mice, although themselves reared 
under identical temperature conditions with the offspring of cold-room 
mice, presented differences of the same sort as had been brought about in 
their parents through the direct effect of temperature, viz., differences 
in the mean length of tail, foot and ear. Unfortunately the data, as 
Sumner points out, give evidence of considerable heterogeneity in the 
genotypic composition of the population of mice used in the experiment. 
Furthermore no control lots from the same stock of mice were reared 
under average temperature conditions for comparison, and the pregnant 
females were not removed to the common temperature room until 
after the young had been carried in utero for 2 weeks. When, in 
view of these uncertainties, it is learned that in only three out of twenty- 
one cases of statistical comparison of the offspring of warm-room and 
cold-room parents is the actual difference more than 4 times the 
probable error of that difference, it appears that the evidence hardly 
warrants any definite conclusions. The investigation is mentioned here 
in order that the student may realize something of the difficulties in- 
volved in attacking this general problem. 
The Transmission of Functional Modifications.—There finally re- 
mains the question of the transmission of the effects of use and disuse, 
and this in a sense is the field in which most tenacious adherence to the 
doctrine of the inheritance of acquired characters is found. Use and 
disuse was one of the chief factors considered by Lamarck in his 
attempt to account for change in species; use and disuse with Darwin, 
in spite of his open hostility to Lamarckism, was an important factor 
in the evolution of species. Use and disuse was supposed to account for 
the blindness of cave fauna, for the reduction in size of wings of the ostrich 
and emu, for the loss of legs by snakes, and for a host of other similar 
structural changes. 
In animal breeding it is in this category perhaps that the inheritance 
of acquired characters assumes its greatest practical importance. The 
development of speed in race horses has already been referred to, But 
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