4. The causes of variation, first, as between different individuals of the same generation; second, as between different individuals of the same parentage but of different generations; third, as between different individuals of the same parentage and ths same generation, as with twins, litters, etc., without regard to parent age; fourth, as between succeeding generations of the same species, representing deviation of the race; fifth, modifications in the individual during its development plainly due to the condi tions of life, rather than to heredity. These different uses of the term variation should be kept distinctly in mind, and evcn then it is better to use the word "deviation)) for the fourth, and °modification" for the fifth form of what is usually called variation. These causes will drop into two groups; first, selective mating, raising questions touching the basis of selection; sec ond, the environment, raising all questions of the effect of the conditions of life, acclimati zation and the inheritance of modifications or acquired characters.
5. Statistical studies of heredity in order to construct the array, eliminate 'chance, determine the skew, study the laws of regression and pro gression and the relative influence of parents.
6. Correlation, as determined by correlation tables, and useful as indexes of valuable char acters.
7. The practical selection of breeders ac companied by a full appreciation of the finan cial considerations involved, and the relation between performance and breeding powers as exhibited by livestock records.
8. The testing of sires as a final basis of selection.
9. The importance of selecting for prolific strains, among both animals and plants, and the advantages of vigor and longevity.
10. The disturbing effect of fashion and the best methods of meeting its demands in breed ing without sacrificing real quality.
It is important in the study of this phase of evolution that the breeder have in mind fairly comprehensive ideas of general evolu tion; that he be not a blind adherent of any peculiar dogma, and that he free his mind from a mass of traditions that serve only to cloud the judgment and deter progress. For exam ple it has been held as a general principle that inbreeding is necessarily fatal to fecundity and to vigor; yet wheat, one of our most vigorous and prolific crops, is systematically inbred. One of the first lessons the breeder should learn, therefore, is that what is true of one species is not necessarily true of all species.
What has been accomplished by way of im provement, and it is much, has been gained al most exclusively by selection, and under disad vantageous circumstances. For example, among plants increase is so rapid as to compel sweep ing selections merely for reduction of numbers, not affording opportunity for as good a basis of judgment as if more space and time were at hand. In this way some of the best things are lost. On the other hand among animals numbers are too few for the best selection and the oft-repeated attempt "to establish a small herd of high-class animals)) has been as often a failure for the reason that numbers are too few to afford material for proper selection even to maintain the initial standard, to say nothing of improvement.
The well-nigh universal practice of using young sires is fatal to the most rapid progress, for in such cases the selection has been made before full development. Thus some of the worst specimens are accepted because prepos sessing at an early age and many of the best are discarded which if given time to fully ma ture would prove their right to exist.
The practice among the better breeders to day is tending toward larger numbers, better conditions of life, the selection of more ma ture animals, followed by an actual breeding test resulting in older sires, emphasis of those °points° that have their basis in utility, and withal an ideal standard that, once adopted, is changed but slowly, if at all.
As would be expected the most rapid prog less has been made in those breeds or varieties of animals and plants in which practically all the individuals can be put to the performance test. For example, it is comparatively easy to get a record from a cow or a speed horse and to know what the one can do at the pail or the other on the track. On the other• hand it is impossible to put the meat animals to the actual test without sacrificing the individual as a breeder. Accordingly breeders of beef cattle, swine and other meat-producing animals have been working somewhat in the dark and to relieve the situation many have felt obliged to sacrifice on the block some of their best bred animals in order to test their standards of selection and verify their methods of breeding.
The improvement of animals and plants is a difficult and often a money-losing enterprise, but it is fascinating because of what is possible. By breeding, the sugar content of beets has been increased from 3 or 4 per cent to 12 and even 20 or more per cent. Corn has been bred richer in nitrogen than is wheat, and its oil con tent is raised or lowered at will. So far as is known any character may be substantially im proved and the upper limit of improvement has never yet been reached with any animal or plant.
Many plants, and some animals, have re ceived little attention at the hands of the breeder, and have either become extinct or else exist among us with little or no improvement, except such as has naturally followed upon better general conditions. Conspicuous ex amples are clover and alfalfa among legumes; timothy and other non-grain producing grasses; asparagus, salsify and many other vegetables and most shade and ornamental trees.
Cats breed without attention and their varia tions do not, therefore, become fixed. The American bison was allowed to become extinct, not because he would not have become useful if carefully bred, but because he was too near like common cattle to repay the trouble of do mestication. In the same way the common hen prevented the domestication of the prairie chicken, but fortunately no real rival stood in the way of that truly American bird, the turkey.
Thus has thremmatology lost much valuable material, but notwithstanding this there is yet at hand awaiting the attention of the master much that is full of undeveloped possibilities, and with the development of our knowledge of the principles underlying heredity and varia tion great improvements in methods of breed ing may be confidently expected.