48 PHYSICAL BASIS OF CIVILIZATION 



toad-flax, and other varieties of plants, which are 

 stated to have occurred after changes in nutrition, 

 etc., and which are reported to have been trans- 

 mitted by heredity ; and also from the rapidity and 

 ease with which, in lowly organized forms of animal 

 life, alterations in environing conditions are stated 

 to have produced changes in the nature and form 

 of organs and tissues, and in the part of the organism 

 where these occur; heads having been reported as 

 forming where tails usually are, and vice versa. 



When in such cases the environmental changes 

 and the mutations continue for several generations, 

 then it is probable that these modifications were 

 produced in each generation de novo, and therefore 

 is their reappearance then no evidence of hereditary 

 transmission. But if such mutations reappear, 

 after environmental conditions have reverted to the 

 original state, then a probable case of hereditary 

 transmission can be asserted. 



In higher mammalia, structures and functions 

 have reached an orderly degree of complexity, 

 specialization, and co-operation. When in such 

 organisms, therefore, every one of the numerous 

 differentiated structures maintains a nicely adjusted 

 balance with all the others, then the creature can 

 continue in life and health. Small and rare irregu- 

 larities are not without their risks. But greater 

 ones may produce pathological conditions or death. 

 On account of the degree of co-operation between 

 organs on which life and health depend, it is un- 



