22 INTRODUCTION. 



tion called the blade or limb. In the scale, stipule, and 

 bract, the petiole and lamina of the leaf are reduced to rudi- 

 mentary condition ; in the sepal the former is wholly sup- 

 pressed and the latter more or less developed. In the petal 

 both lamina and petiole are sometimes present, as in the 

 pink and wall-flower. The petals of these flowers which are 

 broad and expanded at their summit gradually taper into a 

 narrow stalk or petiole, which in this instance is called an 

 unffuis, or claw. In the stamen, the petiole is represented 

 by the filament, the lamina by the anther, whilst the repro- 

 ductive matter called pollen, which is contained in the 

 anther-cells, is only a peculiar transformation of the paren- 

 chyma of the leaf. In the pistil, there appears to be the 

 greatest departure from the primitive type ; yet it is not 

 diificult to trace it even in this instance. The pistil is 

 nothing but a folded leaf, the margins of which have united 

 to form a placenta or point of attachment for the ovules, 

 or young germs, which develop along its edge. Other in- 

 stances might be brought forward illustrative of the fact 

 that the same laws of development and adaptation, govern 

 the organization of both plants and animals. 



It is only by viewing nature as a whole that any part of 

 nature can be properly understood. It is to the study of 

 comparative anatomy and physiology, that we are indebted 

 for our knowledge of this identity and unity of organization. 

 The organism of one species has been compared with that 

 of another, and the transitions forms of the several organs 

 have been traced, so that organs are now recognized as the 

 same which were formerly thought to be altogether difierent. 

 It is to the contributions which they mutually afford each 

 other that we are indebted for the advance of the physical 

 sciences, and the same principle applies to natural history. 

 Almost every part of the human frame has its homologue 



