THE ORIGIN OF SPECIES. 6i 



"evolution "and "development " have been elaborated 

 by otber thinkers, whose province was less specially 

 that of natural history and more decidedly that of 

 abstract philosophy. We owe to Mr. Herbert Spencer 

 a clear explanation of their underlying idea as being 

 that of the progress of differ entiation, or change 

 from the homologous to the lieterogeneous : a process 

 of growth, that is, by which a simple form gradually 

 initiates the structural details of a complex form. All 

 this is included in our ordinary idea of growth. When 

 we think, for instance, of the growth of a plant — I 

 instance a plant because the progress of its growth 

 is more familiar and accessible to our observation 

 than that of animal forms — we do not think merely 

 of its increase in size, but also of the mysterious 

 though familiar processes that go on concomitantly 

 with the increase in size ; the processes by which the 

 seedling, with its simple structure of a straight stem 

 and a few pairs of leaves, presently opens out into 

 a complicated system of countless branches of which 

 the seed held no trace ; the processes by which the 

 soft indefinite tissue of the growing point of a stem 

 stiffens into new and varied forms of tissue that 

 build up leaf and flower ; the processes by which the 

 flower, losing a certain set of tissues and organs, 

 gives rise to others wholly different, and alters into 

 the fruit, cradle of new organisms destined to pass 

 again through the same cycle of successive change. 

 These processes, which are not merely the unfolding 

 of what was already there, but the production of 



