1899] ANIMAL SYMMETRY 63 



to the entire individual, although the latter end of the series has not 

 yet been completely attained. There is also this parallel, that in the 

 formation of the stereo-symmetric individual only one " form " is evolved, 

 just as occurs in vital molecular phenomena in contradistinction to 

 purely chemical. 



Thus we do not find any human body with a liver on the left side, 

 with stomach and spleen on the right, a right aortic arch and a left 

 azygos vein, and so on, though it would be hard for an anatomist to 

 pronounce that such an enantiomorph of the human subject is an 

 impossibility, and harder still to say why such is not in existence. 



Could an experimental morphologist immensely hasten the processes 

 of evolution there should be no difficulty in subjecting any given 

 organism to a special environment, and, by the inter-action of the two, 

 producing an organism exhibiting any of the three types of centro-, axo- 

 or piano-symmetry. By having recourse to a special environment 

 (separation of the blastomeres) he may even to some extent produce 

 stereo-symmetric organisms, but in equal quantities of right and left- 

 sided individuals, unless one enantiomorph is destroyed purposely in the 

 making. By no process could he produce a single stereo-symmetric 

 " form " without its enantiomorph, because the origin of such lies in the 

 molecular constitution of the organism and has no connection with the 

 environment. 



Though not expressed in chemical language, the analogy of the 

 above to molecular stereo-symmetry is complete. 



The asymmetric lowest type of organism changes its form continu- 

 ally and is the " sport " of its environment ; the centro-symmetric, 

 axo-symmetric, and piano-symmetric form gradations in which the 

 organism becomes more definite and permanent in shape, as it becomes 

 less dependent on its environment and more voluntary in its actions. 

 Lastly, a purely stereo-symmetric organism (with a stereo-symmetric 

 ontogeny), the possible product of future evolution, will have so far 

 freed itself from the effect of its environment that changes in the 

 latter will no more affect its morphological plan (complete heterogeneity). 

 The homogeneity of structure will then have passed from the three 

 dimensions of space to the so-called " fourth dimension " of time. 



The study of animal symmetry points to no conclusion more clearly 

 than the future development and predominance of the stereo-symmetric 

 individual, in which the fundamental stereo-symmetry of the physical 

 unit has extended itself to the morphological stereo-symmetry of the 

 organism. 



The University, 



St. Andrews. 



