INTRODUCTION ix 



probability that the same abnormality can lie artificially produced. The converse proposition carries 

 with it an even stronger degree of likelihood. 



Teratologieal variation is thus not essentially different in kind from ordinary variation. 

 Owing, however, to infrequency or non-survival or absence of reproductive faculty, the major 

 teratological variations will not, as a rule, have any influence or evolution. That this is not 

 always the case may be surmised from the facts relating to hermaphroditism (p. 46), and can 

 be shown to be practically certain from a consideration of the phenomena which are connected 

 with germinal duplicity and multiplicity, that is the production of more than one embryo from 

 a single ovum. Amongst these phenomena we have (a) on the purely teratological side, the 

 occurrence of double or multiple monstrosities in all classes of animals ; (b) intermediate between 

 teratology and normal ontogeny, the occasional production in the higher animals of complete and 

 separate unioval twins or triplets ; (c) in normal ontogeny, the polyembryony which occurs in the 

 development of the edentate mammal Tatusia (p. 38) and the alternation of generations which 

 has taken a definite place in the life-history of various groups of animals ; (d) Lastly, on the 

 experimental side and in illustration of what was said under II. above, it need hardly be pointed 

 out that we have now numerous well-ascertained instances in which double or multiple forms 

 have been artificially produced through the (virtual or complete) separation of individual cells 

 or of cell masses during early stages in the development of a single ovum. 



