134 CHAPTERS ON EVOLUTION. 



leaf-like structures. But such considerations do not affect the 

 general axiom of Goethe that the leaf is the type of the whole plant ; 

 for before stamens and pistils were developed at all, there must have 

 existed leaf-like organs adapted for nourishment, and, as we see in 

 lower plants, exercising reproductive functions as well. Stamens 

 and petals, in other words, are secondary developments ; the leaf 

 remains, as before, the type of the whole plant-kingdom. 



The flower, however, is not the only part of the plant which has 

 received abundant elucidation at the hands of the science of like- 

 nesses. The ingenuity of Nature and the prolific nature of the 

 expedients by which she has developed structures to serve her varied 

 ends, formed of old two of the stereotyped sources of wonder by 

 the recital of which philosophers were wont to regale their auditors. 

 This fertility of device in using simple means to effect important ends 

 receives a new reading from the study of homology. We now perceive 

 that the modifications effected by nature represent the utilisation 

 of like parts in divers ways. Just as essentially similar limbs may 

 be employed in the animal world for very different purposes, so the 

 variations of similar parts in plants may illustrate what is meant by 

 " homoplastic " organs that is, the adaptation to new and varied ways 

 of life, of the common belongings of the plant world. Our com- 

 prehension of this truth may be firstly assisted by an example 

 culled from the animal world. The idea that Nature, " in framing 

 her strange fellows," and in developing the unusual and unwonted, 

 should effect her purpose by the creation of new structures and fresh 

 parts, is an idea for which there apparently exists the warrant of 

 common sense. But let us see if the way of Nature in such a case 

 is not rather by the elaboration and modification of already existing 

 parts. Take as an illustrative case the Tortoise (Fig. 60) and its 

 structure. No single animal form stands apparently more aloof from 

 its neighbours of the reptile class than the sluggish chelonian. En- 

 closed in a bony box, its structure seems to be unique, and its relations 

 to the serpent, lizard, or crocodile extremely unapparent. But what 



has comparative anatomy to 

 say respecting the building of 

 the chelonian house? Look at 

 the roof formed by the greatly 

 expanded ribs and solid spine. 

 Regard its sides formed by the 

 cartilages or ends of the ribs; 

 and its floor formed by certain 

 skin-bones comparable roughly 

 , in their nature to the large 



FIG. 60. TORTOISE. , e ,, j-i > j 



scales of the crocodile s under 

 surface, and in any case presenting us with no structures unusual or 



