350 THE EVOLUTION OF LIFE. 



that a cyclopaedia is needed to describe its constituent parts. 

 The germinal vesicle is so simple that it may be defined in 

 a line. Nevertheless, a few months suffice to develop e the 

 one out of the other ; and that, too, by a series of modifica- 

 tions so small, that were the embryo examined at successive 

 minutes, even a microscope would with difficulty disclose 

 any sensible changes. Aided by such facts, the conception 

 of general evolution may be rendered as definite a concep- 

 tion as any of our complex conceptions can be rendered. If 

 instead of the successive minutes of a child's foetal life, we 

 take successive generations of creatures — if we regard the suc- 

 cessive generations as differing from each other no more than 

 the foetus did in successive minutes ; our imaginations must 

 indeed be feeble if we fail to realize in thought, the evolu- 

 tion of the most complex organism out of the simplest. If a 

 single cell, under appropriate conditions, becomes a man in 

 the space of a few years ; there can surely be no difficulty in 

 understanding how, under appropriate conditions, a cell may, 

 in the course of untold millions of years, give origin to the 

 human race. 



It is true that many minds are so unfurnished with those 

 experiences of Nature out of which this conception is built, 

 that they find difficulty in forming it. Habitually looking 

 at things rather in their statical than in their dynamical 

 aspects, they never realize the fact that, by small increments 

 of modification, any amount of modification may in time be 

 generated. That surprise which they feel on finding one 

 whom they last saw as a boy, grown into a man, becomes 

 incredulity when the degree of change is greater. To such, 

 the hypothesis that by any series of changes a protozoon 

 should ever give origin to a mammal, seems grotesque — as 

 grotesque as did Galileo's assertion of the Earth's movement 

 seem to the Aristotleans ; or as grotesque as the assertion of 

 the Earth's sphericity seems now to the New Zealanders. 

 But those who accept a literally-unthinkable proposition as 



