RETICULAR HEREDITY 



Heredity in a Network of Descent — A Conception Based on the Normal Evolu- 

 tionary Condition of Species— Characters Represent Lines of Descent 

 Rather than Independent Units in Germ-Cells. 



O. F. Cook 

 Bureau of Plant Industry, U. S. Department of Agriculture, Washington, D. C. 



THE practical value of a scientific 

 theory lies in its use as a method 

 ^ of thinking. Study of the general 

 problems of heredity should en- 

 able us to think more clearly regarding 

 the special problems that need to be 

 solved in the interest of agriculture and 

 the racial improvement of mankind. 

 The danger is always that we allow our 

 thinking to be governed by a few facts 

 or by facts drawn from too narrow a 

 field of investigation. Disconnected 

 facts command only a limited interest, 

 but a theory that suggests relations 

 between different kinds of facts often 

 stimulates interest and leads to further 

 discoveries. The utility of a general 

 theory, as well as its probability of 

 truth, is judged by its application to a 

 wide range of facts. 



Assumptions or analogies that may 

 appear thoroughly justified by one class 

 of observations or experiment often 

 need to be modified to bring them into 

 agreement with data of other kinds. 

 In this way there may be a gradual 

 attainment of a better point of view, one 

 that facilitates the study of all of the 

 related facts. Yet these inductive 

 methods of biological investigation often 

 appear very indirect in comparison 

 with the well formulated systems of the 

 "exact sciences." 



The study of heredity is often nar- 

 rowed to a search for a "physical basis" 

 or "mechanism of heredity." The 

 morphological conception of plants and 

 animals as made up of different tissues 

 and organs has been carried over into 

 heredity, and has suggested the idea of 



finding something in the germ-cell to 

 correspond to the characters of the 

 adult. The first investigators looked 

 for a complete model of the next genera- 

 tion in the egg, and many similar hopes 

 of finding mechanism of heredity among 

 the organs of cells have been cherished, 

 all to be abandoned in turn, with the 

 progress of cytological discovery. The 

 sex-chromosome hypothesis, suggested 

 by cytological studies of certain groups 

 of insects, is the most recent of these 

 mechanical theories, but is now chal- 

 lenged by other investigators familiar 

 with the cytology of the same insects. 

 The resrdts of breeding experiments 

 are also at variance with the theory 

 that the sex characters are transmitted 

 in the sex-chromosomes.' 



MECHANISM STILL HYPOTHETICAL. 



If something in the protoplasm of the 

 germ-cell could be shown to stand in a 

 definite causal relation to some featiire 

 of the adult organism, we might begin 

 to understand heredity in this physical, 

 mechanical sense. But as yet we have 

 no such basis of observation to warrant 

 the localization of particular characters 

 in particular parts or organs of the 

 germ-cell. Indeed, we have no knowl- 

 edge of characters except in their 

 external manifestation, as features or 

 differences to which attention may be 

 directed in the study of heredity. 

 Theories of heredity are still useftd 

 only as analogies and have to be judged 

 by their consistency with general bio- 

 logical facts and their convenience as 

 methods of thought, in bringing the 



'Foot, Katharine, and Strobell, E. C. Preliminary Note on the Results of Crossing Two 

 Hemipterous Species with Reference to the Inheritance of an Exclusively Male Character and Its 

 Bearing on Modern Chromosome Theories. Biological Bulletin, 24: 187, 1913. 



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