Dr. Baird on Entozoa, or Intestinal Worms. 143 



the like forces effected the reunion of their elements, and brought 

 about the formation of the simplest kinds of animal organism, 

 and these by aggregation constituted the more complex struc- 

 tures. The modern advocates of spontaneous generation, such 

 as Rudolphi, Bremser, &c, very properly treat this notion as 

 absurd ; but they contend, nevertheless, that the intestinal worms 

 must either enter the bodies of the animals in which they live 

 from without, or that they must spring spontaneously into exist- 

 ence. As Entozoa are found in the unborn foetus of many ani- 

 mals, they cannot, they say, in these cases be produced from 

 without. As they only live in living bodies, dwelling according 

 to the different species in the deepest and most impenetrable 

 parts of the body, they cannot be introduced into those parts 

 from without. When an insect of any kind introduces itself into 

 any part of a body, it is always attended with pain. Entozoa, 

 on the contrary, frequently live in the bodies of animals for a 

 length of time, without letting their existence be known. Many 

 animals have worms peculiar to themselves. If the worms, or 

 their ova, come from without, each animal might possess any 

 one or all of the species. Intestinal worms have a peculiar struc- 

 ture, which we do not see in any other class of animals ; they only 

 live upon an aliment already elaborated or assimilated in living 

 bodies. Solitary hydatids exist, these advocates continue, in the 

 brain for instance, which have no sexual organs ; how then can 

 we suppose them to have the faculty of propagating themselves, 

 or transporting themselves from one body to another ? They 

 conclude by asking, — Is it impossible that mucous particles 

 existing in the humours of the animal body might approach each 

 other, unite, and arrange themselves in the form of worms by" 

 the vital powers of the organs, in places favourable for their pro- 

 duction, and finish by becoming animated living beings ? Ana- 

 logy, however, is strongly opposed to this doctrine. From the 

 highest orders of the Mammalia down to the minutest insects 

 which our microscopes enable us to investigate, all nature pro- 

 tests against it ; and our knowledge with regard to the origin of 

 those animals we have been able to examine affords a strong pre- 

 sumptive proof, that if we could, by more perfect means of ex- 

 amination, follow up their history, we should find that these 

 creatures, obscure though they be, are produced and multiplied 

 like all other animated beings. 



Needham,in the early part of the last century, advocated another 

 theory. In studying the natural history of the Infusory Animal- 

 cula, seeing these creatures produced in myriads in situations 

 where he did not suppose ova could exist, he conjectured that 

 their appearance could only be accounted for by vegetation, or by 

 what he called a "vegetative force/' This he conceived to exist 



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