APRIL 19, 1912] 
the normal range of variation of this char- 
acter if the selection was discontinued. 
De Vries has pointed out that natural 
selection can produce races and maintain 
them, but its power to develop races be- 
yond the natural range of variability re- 
mains to be demonstrated. 
With reference to his experiments with 
the potato beetle Tower states: 
It is demonstrated that among the fluctuating 
variations there are individuals which are able to 
transmit their particular variation and give rise by 
selection to a race, while the majority are not able 
to hand on their particular conditions to their 
progeny. Races developed by selection from such 
variations have not been carried beyond the normal 
limit of variability of the species. 
These races or selected strains maintain 
themselves as long as the selection is con- 
tinued, and when the selection is discon- 
tinued rapidly regress to the mean of the 
species. 
The above examples from the sugar beet, 
corn and potato beetle will illustrate the 
type of improvement usually secured by 
practical breeders. By their selection they 
maintain a strain of high efficiency without 
having in general exceeded the limits of 
variation of the species or race and without 
having produced new unit characters which 
would be maintained without selection and 
segregate as pure units following hybrid- 
ization. 
Our different breeds of dairy animals are 
maintained in a state of high productivity 
by continuous selection. Cows are followed 
carefully with reference to their milk-pro- 
ducing capacity and their ability to trans- 
mit this quality to their offspring. The 
ability of bulls to beget high milk-pro- 
ducing daughters is taken as a test of their 
value. There can be no doubt, the speaker 
believes, that this selection within the 
breed maintains the breed in a state of 
high efficiency and is absolutely necessary 
SCIENCE 
603 
to the success of dairying. Strictly speak- 
ing, in the course of this selection, however, 
no new type has been produced. It is well 
recognized that the continuous selection is 
necessary to the maintenance of high milk- 
producing capacity, and if the selection 
were discontinued the average milk pro- 
duction of any dairy herd would rapidly 
decline until it reached the normal mean 
for the breed concerned. The same can 
not be said, however, of the breed or race 
characters, that is, those characters which 
distinguish the breeds or races from other 
breeds. Selection is not necessary to main- 
tain the general characters of the Holstein 
breed for, as long as it is not crossed with 
other breeds, it will in general maintain its 
characters so far as color, conformation, and 
dairy type are concerned. The same may 
be said of any of our breeds of cattle and 
horses. The high efficiency of our race 
horses is maintained by the most careful 
selection and yet probably in most cases no 
distinctly new character is added, which 
would maintain itself as a unit character in 
inheritance. 
_ It is true that we are dealing here with 
complex phenomena and limited exact ex- 
perimentation, and a distinct mutant in the 
direction of high efficiency might occur at 
any time and be chosen for breeding which 
would maintain itself without continuous 
selection. 
It is interesting at this point to recall one 
of the most common differences between 
plant and animal breeding which is seldom 
clearly recognized by practical breeders. 
Plant breeders most commonly strive to 
produce new races or breeds with dis- 
tinetive characters which will reproduce 
their desirable qualities without continuous 
selection; while animal breeders almost 
wholly limit their attention to selection 
within the breeds already established, to 
maintain them in the highest state of effi- 
