SECT, iv.] INTRODUCTION. 21 



Such patterns may exist in single cells or in groups of cells, in 

 separate organs or in groups of organs, in solitary forms or in 

 colonies and groups of forms. Patterns which are completed in the 

 several organs or parts will be referred to as Minor Symmetries. 

 These may be compounded together into one single pattern, which 

 includes the whole body : such a symmetry will be called a Major 

 Symmetry. In most organisms, whether colonial or solitary, 

 there is such a Major Symmetry ; on the other hand organisms 

 are known in which each system of Minor Symmetry is, at least 

 in appearance, distinct and without any visible geometrical relation 

 to the other Minor Symmetries. Examples of this kind are not 

 common, for, as a rule, the planes about which each Minor 

 Symmetry is developed have definite geometrical relations to 

 those of the other Minor Symmetries. It is possible, even, that in 

 some if not all of these, the planes of division by which the tissues 

 composing each system of Minor Symmetry are originally split off 

 and differeTitiated, have such definite relations, though by sub- 

 sequent irregularities of growth and movement these relations are 

 afterwards obscured. 



The classification of Symmetry and Pattern need not now be 

 further pursued. The matter will be often referred to in the 

 course of this work, when facts concerning Variations in number 

 and patterns are being given, for it is by study of Variations in 

 pattern and in repetition of parts that glimpses of the essential 

 phenomena of Symmetry may be gained. 



That which is important at this stage is to note the almost 

 universal presence of Symmetry and of Repetition of Parts among 

 living things. Both are the almost invariable companions of division 

 <ind differentiation, which are fundamental characters without which 

 Life is not known. 



The essential unity of the phenomenon of Repetition of Parts 

 and of its companion-phenomenon, Symmetry, wherever met with, 

 has been too little recognized, and needless difficulty has thus been 

 introduced into morphology. To obtain a grasp of the nature of 

 animal and vegetable forms, such recognition is of the first con- 

 sequence. 



To anyone who is accustomed to handle animals or plants, and 

 who asks himself habitually as every Naturalist must how they 

 have come to be what they are, the question of the origin and 

 meaning of patterns in organisms will be familiar enough. They 

 are the outward and visible expression of that order and complete- 

 ness which inseparably belongs to the phenomenon of Life. 



If anyone will take into his hand some complex piece of living 

 structure, a Passion-flower, a Peacock's feather, a Cockle-shell, or 

 the like, and will ask himself, as I have said, how it has come to 

 be so, the part of the answer that he will find it hardest to give, is 

 that which relates to the perfection of its pattern. 



And it is not only in these large and tangible structures that 



