74 REPRODUCTION AND LIFE HISTORY. 



Finally, it may be noticed in connection with heredity, 

 that there is great doubt to what extent the " body "' can 

 definitely influence its own reproductive cells. Animals 

 acquire individual bodily peculiarities in the course of their 

 life, as the result of what they do or refrain from doing, or 

 as dints from external forces. The " body " is thus changed, 

 but there is much doubt whether the reproductive cells 

 within the " body " are affected specifically by such changes. 

 Weismann denies the transmissibility of any characters 

 except those inherent in the fertilised egg-cell, and there- 

 fore denies that the influences of function and environment 

 are, or have been, of direct importance in the evolution of 

 many-celled animals. Such influences affect the body, and 

 produce what are technically called " modifications" but these 

 modifications do not affect the reproductive cells at least 

 not in a specific representative way. Therefore modifica- 

 tions are not likely to be transmitted, and there seems no 

 good evidence to show that they are. Many of the most 

 authoritative biologists are at present of this opinion. On 

 the other hand, many still maintain that profound changes 

 due to function or environment may saturate through the 

 organism, and affect the reproductive cells in such a way 

 that the changes or modifications in question are in some 

 measure transmitted to the next generation. The question 

 remains under discussion, but the probabilities are strongly 

 against the transmissibility of acquired characters. 



It is important to try to distinguish different modes of 

 hereditary resemblance. The characters of the two parents 

 may be blended in the offspring, or those of one parent 

 may find predominant expression (exclusive inheritance), or 

 the characters of one parent may be expressed in one part 

 of the offspring and those of the other parent in another 

 (particulate inheritance). 



Another important inquiry is into the share that the 

 various ancestors have on an average in forming any in- 

 dividual inheritance. The inheritance of an animal repro- 

 duced in the ordinary way is always dual, partly maternal 

 and partly paternal, but through the parents there come 

 contributions from grandparents, etc. Galton's Law of 

 Ancestral Inheritance states that "The two parents con- 

 tribute between them, on the average, one half of the 



