90 



THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



reproductive cells — at least not in a specific representative 

 way. Therefore modifications are not likely to be trans- 

 mitted, 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 environ- 

 ment 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 dis- 

 cussion, 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 indi- 

 vidual 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 total 

 heritage ; the four grandparents, one quarter ; the eight 

 great-grandparents, one eighth, and so on." 



Mendelian inheritance. — Of the greatest practical and 

 theoretic importance in the study of heredity are the laws 

 discovered by Mendel in 1865, but almost ignored until 

 1900. In their original form these laws are empirical 

 formulations of the average results of breeding experi- 

 ments ; but since 1900 the hypothetical basis suggested 

 by Mendel to explain these laws has become much more 

 concrete and definite. 



If black Andalusian and white Andalusian fowls be 

 bred together, the offspring (the first filial or " Fj " 

 generation) bear a finely divided pattern of black-and- 

 white markings which gives a blue effect. But if two of 



