118 MORPHOLOGICAL DEVELOPMENT. 



through the three axes that join the centres of the surfaces, 

 let us limit our attention to the three kinds of bilateralness 

 which here concern us. The first of these is triple 



bilateral symmetry. This is the symmetry of a figure having 

 three axes at right angles to one another, through each of 

 which there passes a single plane that divides the aggregate 

 into corresponding halves. A common brick will serve as an 

 example ; and of objects not quite so simple, the most familiar 

 is that modern kind of spectacle-case which is open at both 

 ends. This may be divided into corresponding halves along 

 its longitudinal axis, by cutting it through in the direction 

 of its thickness or by cutting it through in the direction of 

 its breadth ; or it may be divided into corresponding halves 

 by cutting it across the middle. Of objects which 



illustrate double bilateral symmetry, may be named one of 

 those boats built for moving with equal facility in either di- 

 rection, and therefore made alike at stem and stern. Ob- 

 viously such a boat is separable into equal and similar parts 

 by a vertical plane passing through stem and stern ; and it is 

 also separable into equal and similar parts by a vertical plane 

 cutting it amidships. To exemplify single bilateral 



symmetry it needs but to turn to the ordinary boat of which 

 the two ends are unlike. Here there remains but the one 

 plane passing vertically through stem and stern, on the op- 

 posite sides of which the parts are symmetrically disposed. 



These several kinds of symmetry as placed in the fore- 

 going order, imply increasing heterogeneity. The greatest 

 uniformity in shape is shown by the divisibility into like 

 parts in an infinite number of infinite series of ways ; and 

 the greatest degree of multiformity consistent with any 

 regularity, is shown by the divisibility into like parts in 

 o:ily a single way. Hence, in tracing up organic evolution 

 as displayed in morphological differentiations, we may ex- 

 pect to pass from the one extreme of spherical symmetry, 

 to the other extreme of single bilateral symmetry. This 

 expectation we shall find to be completely fulfilled. 



