PURE LINES AND SELECTION 



Practical Idea of Stock Breeder and Theoretical Ideal of Biologist Differ — 

 Evidence Advanced in Support of "Pure Lines" Inadequate — Variation 

 in Asexual Reproduction — Experience With Hooded Rats Shows 

 Selection Possible in Either Direction. 



W. E. Castle, 

 Professor of Zoology in Harvard University and Research Associate of the Carnegie 



Institution of Washington. 



CURRENT discussions among 

 students of heredity about pure 

 lines of animals and plants are 

 likely to be very perplexing to 

 those breeders of live-stock who pride 

 themselves upon the excellence and 

 purity to breed-standards of the animals 

 which they keep. "Selection within 

 the pure line," we are told by the biol- 

 ogist, "is without effect." But selec- 

 tion is the foundation principle of the 

 stock breeder and a statement addressed 

 to him that selection is useless in the 

 breeding of any sort of domesticated 

 animal, however pure, woiild be received 

 with incredulity. 



It may be worth while therefore to 

 point out that the biologist, as quoted 

 above, and the stock breeder are talking 

 about ^'ery different things when they 

 speak of "pure lines." The stock 

 breeder has in mind a race of animals 

 bred as closely as possible to a recog- 

 nized standard. But he realizes that 

 some individuals conform more closely 

 than others to the standard of the 

 breed, and his experience indicates that 

 continual selection of the "some" rather 

 than the "others" is essential to main- 

 tain the purity of the breed. He under- 

 stands the biologist's dictum that "se- 

 lection within the pure line is without 

 effect" to mean that one animal is as 

 good as another to breed from provided 

 both belong to the same pure race, and 

 his comment is "Nonsense", and he is 

 quite right too. 



But that is not the biologist's mean- 

 ing. The biologist's "pure line" is an 

 imaginary thing. I doubt very much 

 whether it was ever realized in anv 



actual race of animals or plants. It 

 has no more relation to actual animals 

 and plants than a mathematical circle 

 has to the circles described by the most 

 accompHshed draftsman. All the circles 

 of the draftsman have wiggles in them, 

 if you look at them carefully enough; 

 only the mathematician's imaginarv 

 circle is perfect. Now the biologist 

 undertakes to be the mathematician of 

 breeding and to construct an "exact" 

 system of heredity in which the "piu-e 

 line" concept holds a conspicuous place. 

 He reasons thus: Individuals of the 

 same parentage differ for two reasons, 



(1) because of inherited differences and 



(2) because of differences in environ- 

 ment. If differences of environment 

 (soil and situation of plants, care and 

 food of animals) could be altogether 

 eliminated, then animals of the same 

 heredity should be identical, and should 

 produce only offspring like themselves. 

 They would constitute a pure line. 



THE biologist's REASONING. 



The biologist reasons further: Al- 

 though it is impossible to control the 

 environment completely and thus make 

 it uniform and so eliminate its effects 

 upon variation, nevertheless the effects 

 of environment are not inherited. Con- 

 sequently, if two germ-cells could be 

 brought together in fertilization to 

 form an individual, each of which germ- 

 cells was identical with the other in its 

 heredity, a pure individual would result, 

 so far as heredity is concerned, all of 

 whose germ-cells would transmit the 

 same inherited characteristics, and all 

 would look alike except as they were 



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