52 INTRODUCTION. 



marked, that the larval forms of different species bear to one 

 another a far stronger resemblance than exists among adults, 

 the distinguishing characters of the latter being only evolved in 

 the course of their development ; and every new discovery in 

 this direction only gives fresh confirmation to the great law of 

 development early detected by the sagacity of Von Baer, that the 

 more special forms of structure arise out of the more general, and this 

 by a gradual change. The meaning of this law will become ob- 

 vious hereafter, when some of the principal cases to which it 

 applies shall have been brought in illustration of it (Chap. XII). 

 A still more curious series of discoveries has been made, by 

 means of the Microscope, in regard to the early development of 

 the Medusan Acalephs (jelly-fish, &c.), and the relationship that 

 exists between them and the Hydraform Zoophytes ; two groups 

 of animals, which had been previously ranked in different classes, 

 and had not been supposed to possess anything in common. 

 For it has been clearly made out by the careful observations of 

 Sars, Siebold, Dalyell, and others, that those delicate arborescent 

 Zoophytes, each polype of which is essentially a Hydra, not only 

 grow by extending themselves into new branches, like plants, 

 sometimes also budding off detached gemma?, which multiply 

 their kind by developing themselves into Zoophytic forms like 

 those whence they sprang ; but also produce peculiar buds hav- 

 ing all the characters of Medusa*, which contain the proper gene- 

 rative organs of the Zoophyte, but which, usually detaching 

 themselves from the stock that bore them, swim freely through 

 the ocean as minute jelly-fish, without exhibiting the slightest 

 trace of their originally attached condition. The Medusae in due 

 time produce fertile eggs; and each egg developes itself, not into 

 the form of its immediate progenitor, but into that of the Zoo- 

 phyte from which the Medusa was budded off. And thus a most 

 extraordinary alteration of forms is presented, between the Zoo- 

 phyte, which may be compared to the growing or vegetating stage 

 of a Plant (its polypes representing the leaf-buds), and the 

 Medusa, the development of which marks its flowering stage. 

 So again, from the investigation of the early history of those 

 larger forms of "jelly-fish" with which every visitor to the sea- 

 coast is familiar, it has been rendered certain that they too are 

 developed from Polype larvae, usually of very minute size, which 

 give off Medusa buds ; so that whilst they are best known to us 

 in their Medusan state, and the Hydraform Zoophytes in their 

 polypoid state, each of these groups is the representative of a 

 certain stage in the life-history of one and the same tribe of 

 these curious beings, which, when complete, includes both states, 

 as will hereafter be shown in more detail (Chap. XI). Changes 

 very similar in kind, and in many respects even more remark- 

 able, have been found by microscopic inquiry to take place 

 among the Entozoa (intestinal worms) ; but being interesting 

 only to professed Naturalists and scientific Physiologists, they 

 scarcely call for particular notice in a treatise like the present. 



