THE ADYANCEMENT 01' SCIENCE. 71; 



to the parts so called ia Vertebrates. A most extensive field of reform is becom- 

 ing open to the bomogist in that which is essential to the exactitude of his science 

 -^a nomenclature equivalent to express his conviction of the dififerent relations of 

 similitude. Most difficult and recondite are the questions in face of which the 

 march of Homology is now irresistibly conducting the philosophic observer. Sucb 

 for instance, as the following : — Are the nervous, muscular, digestive, circulating 

 and generative systems of organs more tham 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 connexions, rather than by the 

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

 logy 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 signi- 

 ficant, 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 merely been confirmed by Von Baer's later developmental re'^ 

 searches. 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 

 Durkheira and Macleay, by the light of anatomy, 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," &c. John Hunter first enunciated the genei'al 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 multiplied as far as it can 

 be divided into distinct parts ; the other, that certain of those parts become re- 

 productive 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 experi- 

 ments of Trembley on the freshwater polype, those of Spalanzani on the Naids, 

 and those of Bonnet on the Aphides had brought to light the phenomena of pro- 

 pagation by fission, 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 

 vivum ab ovo," appears to be Hunter's proposition of the dual principle above quoted. 

 The experiments of Uedi, Malpighi and others had progressively contracted the 

 field to which the "generatio aquivoea" could with any plausibility by applied. 

 The stronghold of the remaining advocates of that old Egyptian doctrine was tlie 

 fact of the development of parasitic 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 pre- 

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

 and iu suggesting that the Trichina spiralis of the human muscular tissue might 



