ACQUIRED CHARACTERS IN ANIMAL BREEDING 491 



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 tunes 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 



