xvi THE HISTORY OF ZOOLOGY 653 



while under the head of Vermes are included all the phyla recognised 

 at the present day, except Chor^ata and Arthropoda. 



Other naturalists of the eighteenth century must be briefly 

 referred to. Bonnet introduced the idea of a " scale of beings " 

 (echelle des etres), conceiving the true classification to be a linear 

 one, passing in a single series from the lowest to the highest forms. 

 This conception was opposed by Pallas, who introduced the true 

 conception of representing the relationships of the various groups 

 under the form of a much-branched tree. Spallanzani made 

 numerous investigations on reproduction, and, together with 

 Bonnet, Buffon, and Haller, strongly supported the doctrine of 

 preformation already referred to. Haller summed up the 

 position by stating emphatically that there was no such thing as 

 development or differentiation, no part of the body being made 

 before another, but all parts simultaneously created. It followed, 

 as a natural corollary from this view, that the germ destined to give 

 rise to an animal i.e., the ovum according to the ovulists, the 

 sperm according to the spermatists contained within itself the 

 germ of the next generation, this of the next, and so on, ad 

 infinitum, so that the first created male or female of each species 

 contained within its sperms or ova the germs of all future genera- 

 tions, enclosed one within the other, like a nest of Chinese boxes. 

 Buffon, as the result of numerous experiments, came to the 

 conclusion that the ovary secretes a seminal fluid containing moving 

 particles analogous to sperms, and, from this erroneous observation, 

 framed a theory which is an interesting anticipation of Darwin's 

 Pangenesis (p. 642) namely, that organic particles, derived from 

 all parts of the body, occur in the seminal fluids of the two sexes, 

 and that the union of these in the uterus " determines them to arrange 

 themselves as they were in the individuals which furnished them." 



The theory of preformation (as then understood) was practically 

 demolished, and that of epigenesis, or new formation, established 

 on a firm basis, by Caspar Friedreich Wolff, who at the age 

 of twenty-six in 1759 gave the most accurate account of the 

 development of the Chick hitherto known, and showed clearly 

 that there was no preformation of the various parts, but a gradual 

 differentiation from a layer of organised particles, or, as we should 

 now say, from a cellular blastoderm. 



Another great eighteenth century name is that of John Hunter, 

 the most profound comparative anatomist and physiologist of his 

 time. He was not a zoologist in the narrow sense of classifier, 

 but his exquisite investigations on the various systems of organs 

 and their functions throughout the animal kingdom furnished the 

 science with a foundation of wide and exact knowledge which was 

 of far more importance than the most cunningly devised system of 

 classification. Important anatomical investigations were also 

 made during this period by Vicq d'Azyr, who enunciated the 



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