316 DOMESTICATED ANIMALS AND PLANTS 
Survival of the fittest. The triumph of those individuals which are able to 
compete successfully with the environment. 
Testis. The organ in which the spermatozoa or male reproductive cells 
develop. Synonymous with testicle. Plural, testes. 
Thoroughbred. The name of the [nglish running horse, often improperly 
used for pure bred. 
Thremmatology. The science and practice of improving domesticated ani- 
mals and plants. 
Throwing back. Synonymous with reversion. 
Type. The most usual form of a character or of an individual; also used for 
the form that is desired in attempted improvement. 
Unit character. The smallest element that physiologically tends to behave 
more or less independently in the formation of plant or animal characters. 
Unregistered stock. Animals that are undoubtedly of pure breeding but 
whose pedigrees have been neglected or lost. 
Use and disuse. The effect of use or disuse upon the development of an 
hereditary character and its consequent retention or loss by the individual 
or race. : 
Uterus. The female part in which the embryo develops and from which it 
separates at birth. Synonymous with womb. 
Variation. The fact that individuals of the same race and breeding are not 
alike, but differ more or less among themselves. 
Zygote. That portion of the gamete which determines a unit character. 
