MECHANICS AND USEFUL ARTS. 27 



ative systems of organs more than functionally similar in any two primary 

 provinces of the animal kingdom? Are the homologies of entire systems to 

 be judged of by their functional and structural connections, rather than by 

 the plan and course of their formation in the embryo? It may be doubted 

 if embryology alone is decisive of the question, whether homology can be 

 predicated of the alimentary canal in animals of different primary groups or 

 provinces. It is significant, however, of the lower value of embryological 

 characters, to note that the great leading divisions of the animal kingdom, 

 based by Cuvier on Comparative Anatomy, have nearly been confirmed by 

 Von Baer's later developmental researches. And so, likewise, with regard 

 to some of the minor modifications of Cuvier's provinces, the true position 

 of the Cirripeda was discerned by Straus Durkhehn and Macleay, by the 

 lightanatomy, before the discovery of their metamorphoses by Thomson. 

 If, However, embryology has been over-valued as a test of homology, the 

 study of the development of animals has brought to light most singular and 

 interesting facts, and I now allude more especially to those that have been 

 summed up under the term "Alternate-generation," "Parthenogenesis," 

 " Metagenesis/' etc. John Hunter first enunciated the general proposition, 

 that " the propagation of plants depended on two principles, the one that 

 every part of a vegetable is ' a whole/ so that it is capable of being multi- 

 plied as far as it can be divided into distinct parts ; the other, that certain of 

 those parts become reproductive organs, and produce fertile seeds." Hunter 

 also remarked, that " the first principle operated in many animals which 

 propagate their species by buds or cuttings ; " but that, whilst in animals, it 

 prevailed only in "the more imperfect orders," it operated in vegetables 

 "of every degree of perfection." The experiments of Trembly on the 

 freshwater polype, those of Spakinzani on the Naiads, and those of Bonnet 

 on the Aphides, had brought ta light the phenomena of propagation by fis- 

 sion, and by gemmation or buds, external and internal, in animals to which 

 Hunter refers. Subsequent research has shown the unexpected extent to 

 which Hunter's first principle of propagation in organic being prevails in 

 the animal division. But the earliest formal supercession of Harvey's 

 axiom, " omne vinim ab oro," appears to be Hunter's proposition of the dual 

 principle above quoted. The experiments of Redi, Malpighi, and others, had 

 progressively contracted the field to which the " generatio cequivoca,' could 

 with any plausibility be applied. The stronghold of the remaining advo- 

 cates of that old Egyptian doctrine was the fact of the development of para- 

 sitic animals in the flesh, brain, and glands of higher animals. But the 

 hypothesis never obtained currency in this country; it was publicly opposed 

 in my " Hunterian Lectures," by the fact of the prodigious preparation of 

 fertile eggs in many of the supposed spontaneously developed species ; and 

 in then suggesting that the Trichina spiralis of the human muscular tissue 

 might be the embryo of a larger worm in course of migration, I urged that 

 a particular investigation was needed for each particular species. 



Among the most brilliant of recent acquisitions to this part of Physiology, 

 have been the discoveries which have resulted from such special investiga- 

 tions. Kuchenmcister and Von Siebold have been the chief laborers in this 

 field. After noticing some of the results of those labors, he said: Since 

 the time when it was first discovered that plants and animals could propa- 

 gate in two ways, and that the individual developed from the bud might 

 produce a seed or egg, from which also an individual might spring capable 

 of again budding, since this alternating mode of generation was observed, 



