DOMESTIC ANIMALS 



29 



mals. Other investigators have recorded 

 further facts of variation until we now 

 have available a great mass of such 

 material. Eecently a tendency has de- 

 veloped to measure these variations and 

 to express them in mathematical terms 

 and formulae. We may proceed first to 

 a discussion of the causes of variation. 

 At the very outset we must admit that 

 it is practically impossible to conceive 

 of the absolute identity of two such com- 

 plicated structures as farm animals. 

 No two eggs from the same female can 

 be identical. They are matured at 

 slightly different times and are formed 

 of different particles of organic sub- 

 stances which in turn were received 

 from the blood at different times, and 

 from different sources. It is impossible, 

 therefore, for a female animal to produce 

 two identical offspring from different 

 eggs even if both eggs could be ferti- 

 lized by one and the same spermatozoon 

 — 'another impossibility. For the same 

 reason no two spermatozoa can be ex- 

 actly the same. This hereditary varia- 

 tion is ascribed by Weismann to the di- 

 rect action of external influences on the 

 biophors or "life bearers" of the egg and 

 spermatozoon. 



Sex influenced by feed — Now, with- 

 out entering as yet into the subject of 

 determination of sex in animals, it may 

 be remarked that according to numer- 

 ous experiments the quality of nutri- 

 tion exercises some influence in deter- 

 mining the sexuality of the germ cell 

 both before and after fertilization. That 

 inequalities in nutrition exercise an in- 

 fluence on the germ cells must be as- 

 sumed to be true, but the exact nature 

 of the influence is not exactly known. 

 According to Vernon it appears that a 

 highly nourished egg as compared with 

 one receiving less nutriment is more 

 likely to develop into a female. The 

 general nutritive condition of the ani- 

 mal exercises a favorable or unfavorable 

 influence on the germ cells according as 

 the animal is excessively poor or fat. 

 Undue accumulation of fat about the 

 ovaries and testes is unfavorable to the 

 best development of the eggs and sper- 

 matozoa. 



Vigor of offspring — Considerable 

 seasonal variation occurs in the size 

 and vigor of offspring. Thus for exam- 

 ple it seems undesirable to breed sheep 

 during the hot weather of summer, 

 even where it is possible as is the case 

 with Dorsets. The unfavorable influ- 

 ence of hot weather may be exercised 

 throughout the period of pregnancy 



but it is doubtless active to some ex- 

 tent at the time of impregnation and 

 just previously. Professor Ewart has 

 observed that rabbits fertilized before 

 the eggs are mature are likely to pro- 

 duce young which resemble the male, 

 while, if fertilization takes place late in 

 the period of heat, the offspring partake 

 more of the characters of the mother. 

 It is evident, therefore, that the charac- 

 ters of the progeny are in some degree 

 related to the condition of the germ cells 

 at the time of fertilization. 



Parental tendencies— In animals 

 like swine and hare which bear several 

 young at each birth, the individuals of 

 the same litter may differ as greatly 

 as those of different litters from the 

 same parents. During gestation all the 

 fetuses of a given litter are presum- 

 ably subjected to as nearly identical 

 conditions as it is possible to obtain. 

 The differences which appear in the 

 young at the time of birth are hence 

 largely predetermined in the germ cells. 

 According to Weismann the characters 

 of the offspring are almost entirely de- 

 termined at the time of union of the 

 germ cells. The great variation observed 

 m offspring of the same parents would 

 thus be due to unlimited possible per- 

 mutations in the mixing of ancestral 

 tendencies from both parents. 



Size influenced by feed — The only 

 instance in which the offspring do not 

 differ considerably is in the case of 

 identical twins which have been studied 

 by Galton and Weismann and are prob- 

 ably developed from a single egg and 

 sperm cell. But even identical twins 

 differ slightly in some points and this 

 indicates clearly that differences in 

 feed and environment are not without 

 effect during the development of the 

 animal. Convincing evidences of this 

 fact are familiar to every observing 

 stockman. If the milk of the mother is 

 defective in quantity or quality, pigs 

 and lambs may be so stunted in early life 

 that they never attain their normal de- 

 velopment. Chickens improperly or in- 

 sufficiently fed during the first week may 

 never reach more than half their normal 

 size. The size and character of the vital 

 organs, strength of the bones and even 

 the quality of the meat are greatly in- 

 fluenced by the kind of ration. Excess- 

 ive infestation of cbickens with lice and 

 cattle with ticks stunts growth. Tick 

 infested cattle at maturity are 200 

 pounds lighter than other cattle of the 

 same breed which are free from ticks. 

 Moreover, infestation with ticks may de- 



