JULIAN HUXLEY 
the third is the human or psychosocial sector, operating by 
mind-accompanied psychosocial pressure superposed upon 
natural selection, and resulting in human societies and their 
products—from machines and works of art to sciences and 
religions. 
On this earth (and presumably in a few other isolated spots 
in the universe) there have thus been two critical points in 
evolution, when it has entered on a new phase, with new pro- 
perties and characteristics. The first was when, thanks to the 
evolution of deoxyribonucleic acid (DNA) and genes, material 
organizations became self-varying and self-reproducing, and 
the biological phase began to operate. The second was when, 
thanks to the evolution of conceptual thought, symbolic lan- 
guage and the cumulative transmission of experience by 
tradition, mental or mind-accompanied organizations became 
self-varying and _ self-reproducing, and the human _ phase 
emerged. 
In the psychosocial phase, the process of evolution is pre- 
dominantly cultural. Its results are manifested in the variety 
of societies, and of their organs, like philosophies, legal codes, 
and social systems. 
In this phase a new mechanism for securing continuity and 
change has been added. In addition to the biological basis 
of inheritance and variation provided by the gene-complex in 
the chromosomes, man has a cultural basis, in the shape of the 
complex of ideas, beliefs and purposes and their transmissible 
results which is broadly called tradition. With its aid, he can 
accomplish something impossible to any other organism—he 
can transmit experience cumulatively down the generations and 
incorporate its results directly into the evolutionary system. 
In cultural evolution, there is no sharp distinction between 
germ-plasm and soma, between genetic basis and its phenotypic 
results. True, the main stream of tradition is constantly shed- 
ding some of its pathological and lunatic fringe, just as the main 
stream of material germ-plasm is constantly shedding some of 
its pathological and unhelpful mutants; but in general, culture 
is simultaneously manifested and transmitted. 
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