INTRODUCTION. 13 



portant organs of the adult, assist in forming the bud. Into the 

 details of the process, which require in many points a fuller 

 elucidation, it is not my purpose to enter. 



Gemmation is a far commoner occurrence amongst the 

 simpler than amongst the more highly organised forms. It 

 appears to have been superadded to the sexual mode of repro- 

 duction quite independently in a number of different instances. 



While there is no difficulty in understanding how gemmation 

 may have started in such simple types as the Ccelenterata, the 

 manner in which it first originated in certain highly organised 

 forms, as for instance the Ascidians, is somewhat obscure, but it 

 seems probable that it began with the division of the developing 

 germ into two or more embryos, at a very early stage of growth. 



Such a division of the germ is, as has been shewn by 

 Kleinenberg, normal in Lumbricus trapezoides 1 and Haeckel 

 has shewn that an artificial division of the germ in the Siphono- 

 phora leads to the development of two individuals. It has been 

 pointed out by various naturalists that the production of double 

 monsters is often a phenomenon of the same nature. While it 

 is next to impossible to understand how production of a bud 

 could commence for the first time in the adult of a highly 

 organised form, it is not difficult to form a picture of the steps 

 by which the fission of the germ might eventually lead to the 

 formation of buds in the adult state. 



The coexistence of sexual reproduction with normal asexual 

 multiplication, or with parthenogenesis, has led to a remarkable 

 phenomenon in the animal kingdom known as alternations of 

 generations". 



For the details of the various types of alternations of 

 generations, and their origin, the reader is referred to the body 

 of the work ; but a few general remarks on the nature and origin 

 of the process, and on its nomenclature, may conveniently be 

 introduced in this place. The simplest cases are those in which 



1 The case of Pyrosoma, which might be cited in this connection, is probably 

 secondary. 



2 For an excellent account of this subject, vide Allen Thompson's article Ovum in 

 Todd's Cyclopedia. The metamorphosis of the Echinoderms included under this 

 head in Thompson's article is now known not to be a proper case of alternations of 

 generations. 



