THE WONDER OF LIFE 613 



also points out that if the Rotifers were subjected to the 

 alcohol solution indefinitely, generation after generation, the 

 race would probably become extinct. The alcohol lessens 

 the rate of reproduction and it may, in the course of time, 

 progressively weaken the germ-ceUs. What was proved, 

 however, was that ' if the alcohol is removed it is possible 

 for the race to recover and to regain its normal condition 

 in two generations, thus showing that the germ-substance 

 is not permanently affected by the alcohol ' so far as the 

 experiments went. In any case we have a good instance 

 of the stability of the germ-plasm. 



It is only fair, however, to cite a case on the other side, 

 indicating susceptibihty. Dr. Charles R. Stockard made 

 experiments for three years in intoxicating male guinea- 

 pigs by inhalation of alcohol (which does not spoil their 

 stomach), and reached the important conclusion that an 

 alcohohzed male guinea-pig almost invariably begets 

 defective offspring even when mated with a vigorous 

 normal female. The effects were manifest in the second 

 generation animals as well. ' The poison injures the cells 

 and tissues of the body, the germ-cells as well as other 

 cells, and the offspring derived from the weakened or 

 affected germ- cells have all of the cells of their bodies 

 defective.' 



Dentition of Shrews. — ^Let us take a somewhat un- 

 famiUar illustration of a persistent reUc. It is well known 

 that in ordinary placental mammals with various kinds 

 of teeth (heterodont as it is called), there are never 

 more than three pairs of incisors. There is thus a gap 

 between all ordinary mammals and the old-fashioned 

 Polyprotodont Marsupials, such as the Tasmanian Wolf 

 or Thylacine, which has four upper incisors, and the Bandi- 



