62 AN INTRODUCTION TO ZOOLOGY. 



something new that was not already there, except in 

 a potential sense, constitute the " development/"' the 

 " differentiation," the " ontogeny," of that individual 

 plant. These changes are crowded into a short 

 period; we mark their progress in the springtime, 

 day by day, almost hour by hour. But those slow 

 changes, which constitute the growth not of indi- 

 viduals, but of races, kinds, and classes, have, for the 

 measure of their long hours, not the gradations that 

 mark the passage of the shadow on the sundial, but the 

 rise or wreck of continents and the secular displace- 

 ment of the poles. To us, the creatures of a day, they 

 can only be made apparent through the insight of the 

 mind — of such a mind as Darwin's, which by its 

 patient inductions linked the isolated facts given us 

 by the evidence o£ the senses into a bridge that over- 

 steps the bounds of time. To that insight of the 

 mind, the short familiar work of growth is the witness 

 of its larger cycle ; and each tiny germ of life holds, 

 waiting to be read, the secret of the whole universe, 

 and the long history of the organic world. 



Now that we have gained an insight into the pro- 

 cesses by which the different forms of life have been 

 developed, it becomes evident that we have before us 

 an inexhaustible field of discovery. Those processes 

 are still going on, and by careful observation we may 

 hope to see species in the making, to watch the de- 

 velopment of new kinds. It would be impossible to 

 discuss here the various controversies that have arisen 

 as to the causes that lead to what Darwin called varia- 



