WEISMANWS ESSAYS UPON HEREDITY. 3 



discussion of which would be as much out of place 

 as it would be beyond our power to attempt. 



There are two centres round which Professor 

 Weismann's investigations turn a speculation as 

 to the origin of natural death, and a new theory of 

 heredity, and the two are closely connected to- 

 gether, and result in a complete and coherent 

 biological theory. 



i. The first essay raises the question of the reason 

 of the great variation in the duration of life among 

 plants and animals. Can this variation be brought 

 under any law ? It is assumed, to start with, that 

 duration of life depends upon adaptation to ex- 

 ternal conditions, and is governed by the needs of 

 the species rather than of the individual. When, 

 then, the individual becomes useless to the species, 

 whether that stage is reached after a longer or a 

 shorter period, we should expect him to have 

 reached the natural term of his life. And we are 

 confirmed in this expectation by finding that, as 

 a rule, life does not greatly outlast the period of 

 reproduction, except in those species which tend 

 their young. In spite of the fact that there is very 

 little accurate information available as to the dura- 

 tion of life in different species, Professor Weismann 

 is able to produce some remarkable statistics in 

 favour of this view, the most striking perhaps being 

 drawn from the "exceptions" which "prove the 

 rule." But this discussion of the duration of life 



