156 



The Journal of Heredity 



a recognition of the existence of species 

 and an understanding of the organiza- 

 tion of specific groups would seem to 

 be necessary steps in the development 

 of a science of eugenics. To overlook 

 the specific organization of living matter 

 is to leave out of account the funda- 

 mental and unique condition of organic 

 existence and of evolutionary progress. 



The obvious purpose of the study of 

 evolution and heredity is to understand 

 how the complex and highly speciaHzed 

 bodies of plants and animals have been 

 built up, and how they are rebuilt in 

 each generation. From an evolutionary 

 viewpoint, it is apparent that the cellu- 

 lar organizations represented in the 

 bodies of plants and animals have 

 developed through the association and 

 differentiation of cells, and that pro- 

 gress in cellular evolution is dependent 

 upon the organization into groups of 

 interbreeding individuals, the groups 

 that are usually called species. 



Among the lowest forms of life the 

 individual organisms are merely single 

 cells, and the cells are all of one kind, 

 and all of these cells taken together con- 

 stitute the species. Organization, in the 

 sense of building up cellular structures, 

 begins when the cells associate in 

 colonies, instead of hving as separate 

 unicellular individuals. After the habit 

 of living in colonies has become estab- 

 lished, the species is thought of as made 

 up of colony individuals, rather than of 

 cell individuals. In other words, the 

 specific organizations of single-celled 

 organisms furnished the materials for 

 making colonies and building higher 

 types of cellular structures. 



PROGRESS THROUGH DIVERSIFICATION. 



The next stage of organization is 

 reached when the members of a colony 

 of cells are not all of the same kind, but 

 become specialized in form and func- 

 tion. The building up of the higher 

 types of cellular organization involves 

 the association of vast numbers of cells. 

 The s])ceific organization and the cellular 

 organization have made mutual jjro- 

 gress. Sjjccial systems of reproduction 

 have been developed, making it i)ossiblf 



to build up these highl>- complex forms 

 of organization. It has not been suffi- 

 cient to have different kinds of cells to 

 make up individual organisms. Diver- 

 sification has been raised, as it were, to 

 the second power, in that different kinds 

 of polycellular individuals are repre- 

 sented in the specific organizations. 

 Sexual and other forms or diversity 

 among the members of species have 

 developed in all of the higher groups, 

 to accompany increasing specializa- 

 tions of cellular organization.' 



The dependence of the cellular organi- 

 zation upon the specific organization 

 continues and becomes more apparent 

 as we ascend in the scale of the struc- 

 tural complexity, and is shown by the 

 need of crossing of lines of descent 

 through sexual reproduction. The 

 species and the individual are morpho- 

 logical facts, and sexuality the physio- 

 logical function that connects them. The 

 species is a network of interwoven lines 

 of descent and has as real an existence 

 in nature as an individual animal or 

 1^1 ant. The species ])roduces the indi- 

 vidual and the individual adds its share 

 to the network of descent of the species. 

 To think of the individual as producing 

 itself without reference to the specific 

 organization is like assuming the spon- 

 taneous generation of a com])lex cellular 

 structure. 



The true nature of a species as re])re- 

 sented l)y a network of descent is not 

 ajjparent if we consider only the 

 few individuals that may be taken to 

 re])resent the species at a ])articular 

 moment of time. In order to see the 

 network we inust visualize the i)ast and 

 the future, in accordance with the facts 

 of organic succession. Narrowing our 

 \-iew of a species to a short ])eriod of 

 time is like focusing our attention on the 

 cross-section of a fabric. This enables 

 us to examine the free ends of the indi- 

 vidual threads, or to study an occasional 

 knot, but we lose sight of the network 

 as a wliole. 



-rm-: simccies underlies all. 



Tliere is no structural organization 

 witliout underlying specific organiza- 



'Cook, O. F., and Swingle, W. T. Evolution of Cdln! 

 Plant Industry, U. S. Dt-partment of Agriculture, 1905. 



Sti-\uinrcs, Hullrtin SI, Huix'au of 



