CHAPTER XXIV. 



Double Monsters. 



Of the evidence as to double and triple "monstrosity" and 

 of the classification of the various forms no account can be given 

 here. This may be found in any work on general teratology. In 

 this chapter are put together a few notes on points respecting 

 these formations of interest to the naturalist, and having relation 

 to what has gone before. 



It is now a matter of common knowledge that in animals [and 

 plants] division may occur in such a way that two or more bodies 

 may be formed from what is ostensibly one fertilized ovum (cp. 

 multipolar cells). But by a similar division, imperfectly effected, 

 the resulting bodies instead of being complete twins or triplets 

 may remain united together, frequently having a greater or less 

 extent of body in common. In other words, speaking of simple 

 cases in bilateral animals, the whole body, resulting from the 

 development, may contain more than one bilaterally complete 

 group of those parts which normally constitute the Primary Sym- 

 metry of an " individual." 



If well developed, the component groups are most often united 

 by homologous parts, so that there is a geometrical relation of 

 images between the groups together, forming the compound struc- 

 ture, the whole being one system of Symmetry. Concerning the 

 relations of the several parts of such a system to each other 

 numerous questions of interest arise, but with these it is not now 

 proposed to deal. 



To those unacquainted with facts of this class it may be of use to point out in 

 the fewest words the direction in which this importance lies. It arises, briefly, from 

 the fact that in the resemblance between a pair of homologous twins, whether wholly 

 or partially divided, there is once again an illustration of the phenomenon of Sym- 

 metry, and of the simultaneous Variation of structures related to each other as sym- 

 metrical counterparts. 



The frequency of close resemblance between twins is a matter of common know- 

 ledge. If it be true that such twins may result from the development of one ovum — 

 a fact that cannot be doubted in face of the complete series of stages intermediate 

 between total and partial duplicity — the resemblance between these twins is then of 

 the same nature as that subsisting between the two halves of any other bilaterally 

 symmetrical system. A wide held of inquiry is thus opened up. For, as suggested 

 in the Introduction (p. 36) if the very close resemblance of twins to each other is a 

 phenomenon dependent on Symmetry of Division, the less close resemblance between 

 members of families may be a phenomenon similar in kind. 



