716 INHERITANCE 



INHERITED DEGENERATIVE CHANGES RESULTING FROM UNFAVORABLE 

 CONDITIONS 



In many cases, when ciliate Infusoria are cultivated for long periods 

 in isolation cultures, in which great numbers of successive generations 

 are produced, the organisms are found in the later generations to decline 

 in vigor and vitality. This change is progressive; it becomes greater in 

 later generations. The vital processes become "depressed," slow, ineffi- 

 cient; in particular the rate of multiplication decreases. In time the ani- 

 mals become degenerate — abnormal in form and structure, reduced in 

 size. As an index of this decline in vigor and vitality, the changes in the 

 rate of multiplication are commonly employed. Graphs of the daily num- 

 ber of fissions show a curve gradually descending from a high point 

 at the beginning of the isolation culture, to nearly zero at a later period. 

 A large number of such graphs, based on the work of many different 

 investigators on many species, are published in the author's Genetics 

 of the Protozoa (1929). 



It has been held by many investigators that this decline is a matter 

 of age; that these graphs are curves of senescence. The earlier periods of 

 immaturity and maturity were believed to be followed inevitably by a 

 period of senescence. Whether this is, indeed, true for some species is 

 still uncertain. But for a number of species it has been shown that the 

 decline need not and does not occur if the conditions are kept entirely 

 favorable (a summary of investigations on this matter is found in the 

 author's Genetics of the Protozoa, 1929). In these latter species, there- 

 fore, the decline and degeneracy are consequences of life under unfavor- 

 able conditions. 



Thus unfavorable environmental conditions, acting for many succes- 

 sive generations, cause changes in the characteristics of the individuals, 

 and cause them to produce in the later periods offspring that differ from 

 those produced in the earlier periods. In the earlier periods, parents and 

 offspring are vigorous, multiplying rapidly. Later, under the same 

 environmental conditions as before, parents and offspring are weak, 

 nonresistant, multiplying slowly. The effects of the unfavorable environ- 

 ment become cumulative as generations pass, and in vegetative repro- 

 duction they are transmitted to the offspring. 



The inheritance of the depressed condition is demonstrated in the 



