HEREDITY. 255 



physiological units slightly unlike those of the species ; and 

 these slightly- unlike physiological units, communicated 

 through the medium of sperm- cell or germ- cell, will tend, in 

 the offspring, to build themselves into a structure similarly 

 diverging from the average of the species. 



It is not equally manifest, a priori, however, that on this hy- 

 pothesis, alterations of structure caused by alterations of func- 

 tion, must be transmitted, to offspring. It is not obvious that 

 change in the form of a part, caused, by changed action, in- 

 volves such change in the physiological units throughout the 

 organism, that these, when groups of them are thrown off in 

 the shape of reproductive centres, will unfold into organisms 

 that have this part similarly changed in form. Indeed, when 

 treating of Adaptation (§ 69), we saw that an organ modified 

 by increase or decrease of function, can but slowly so re-act 

 on the system at large, as to bring about those correlative 

 changes required to produce a new equilibrium ; and yet only 

 when such new equilibrium has been established, can we ex- 

 pect it to he fully expressed in the modified physiological units 

 of which the organism is built — only then can we count 

 on a complete transfer of the modification to descendants. 

 Nevertheless, that changes of structure caused by changes 

 of action, must also be transmitted, however obscurely, from 

 one generation to another, appears to be a deduction from 

 first principles — or if not a specific deduction, still, a general 

 implication. For if an organism A, has, by any peculiar 

 habit or condition of life, been modified into the form A, it 

 follows inevitably, that all the functions of A', reproductive 

 function included, must be in some degree different from the 

 functions of A. An organism being a combination of 

 rhythmically-acting parts in moving equilibrium, it is im- 

 possible to alter the action and structure of any one part, 

 without causing alterations of action and structure in all the 

 rest; just as no member of the Solar System could be modi- 

 fied in motion or mass, without producing re-arrangements 

 throughout the whole Solar System. And if the organism A 



