18 'Life and Death, Heredity and Evolution 



ity, they do show us life and the passage of generations 

 following a very different course from that which we see in 

 ourselves; a course which opens to us many new vistas. 

 And they do present us a world of life in miniature in which 

 structures are condensed into small space, in which activities 

 are condensed into brief time. In a watch-glass on our 

 table we may in a week see generations come and go; may 

 observe the rise of whole faunas and floras ; their decline and 

 replacement by others; we may follow in successive genera- 

 tions the struggle for existence; may see natural selection 

 at work and discover its results. In a few days we may 

 see the birth, babyhood, youth and age of individuals, and 

 their replacement by descendants ; we may study the inherit- 

 ance of parental traits by the new generation, or the ap- 

 pearance of new traits ; we may observe how the population 

 changes with the passage of ages, and all while we wait 

 for one of the changes of the moon. 



This seems to present a wonderful opportunity for solving 

 in a short time some of the problems at which men have been 

 at work for ages, so that students have taken up this work 

 with enthusiasm. But unhappily, when we cease to regard 

 it as a mere fascinating spectacle, and desire to establish, 

 in the rigid and detailed manner required by science, just 

 what is happening in these teeming populations, and just 

 what are the laws that the events follow, we find the diffi- 

 culties very great, and our success perhaps less rapid than 

 we might hope. For to do this we have to come to know 

 these creatures individually; we have to work with them as 

 we would with guinea pigs or with calves. The distance 

 of size between them and ourselves is almost as difficult to 

 overpass as are the spatial distances between us and the 

 stars, so that studying them individually is a little like try- 

 ing to get acquainted with the inhabitants of Mars. To 



