42 HEREDITY 



the new conditions has gradually, in the course of ages, 

 become hereditary ? 



There is some evidence, too, that the changes produced 

 in mice by rearing at higher temperatures are inherited 

 in a slight degree. And even more striking results 

 have been obtained by causing species of salamanders 

 and toads to lay their eggs in water instead of on land, 

 by which changes in structure are brought about. But 

 further investigation is required to confirm these 

 results. 



There are still other types of modifications which are 

 sometimes regarded as heritable. We frequently hear, 

 for instance, of hereditary disease. Several members 

 of a family die of consumption, and we say that the 

 disease has been inherited. But on examining such a 

 case, it appears that we cannot strictly speak of con- 

 sumption, or in fact of any disease, as hereditary. 

 Consumption is due to a special microbe, which must 

 somehow be introduced into the system before the 

 disease can appear. Sometimes it may be introduced 

 from the mother before the birth of the child, but even 

 in this case we cannot regard the disease as inherited, 

 since all that occurs is the transference of the parasite 

 from one person to another. 



It is known, however, that certain people are much 

 more liable than others to contract certain diseases, 

 and that this liability is frequently strongly inherited. 

 But this liability is an inborn quality, so that its in- 

 heritance has nothing to do with the present question. 



To sum up the main argument, it must be said that 

 there is some presumptive evidence in favour of the 

 inheritance of acquired characters, but that direct 

 experiments have given positive results of only the most 

 meagre and inconclusive kind. Completely negative 

 results have been obtained times out of number, but 

 this of course cannot be taken as negative proof. 



Finally, we have the difficulty of conceiving any 

 mechanism which would bring about the inheritance 

 of modifications. At the same time, a process may exist 

 of which we are entirely ignorant. 



