4 INTKODUCTION. 



morbid processes in any organ were competent to give rise to para- 

 sites, assuming that the elementary constituents of an organ affected 

 by disease, mechanically separated themselves from their natural 

 connection, and not perishing, but transforming themselves into 

 independent organisms, became parasites. Clothed in fine phrases, 

 this idea was everywhere received with favour, and took such deep 

 hold of the public mind, that it is now a matter of no small 

 trouble to eradicate what has, with many, become an article of 

 faith, and to substitute the laws of nature, drawn from experience, 

 for the creation of their fancy. It was certainly more convenient 

 and enticing to give free scope to one's thoughts, and to fill up 

 the frequent gaps left in our knowledge of the origin and multipli- 

 cation of the lower animals, with pure hypotheses, than as now, 

 renouncing this faulty method of inquiry into nature, to attain, 

 by troublesome researches and careful experiments, a secure insight 

 into her hidden workings. 



It was by the latter method that a remarkable and hitherto 

 unanticipated development of the sexual apparatus was discovered 

 in many parasites, such as round-worms, thread-worms, tape- 

 worms, and flukes, 1 in which such an immense mass of eggs 

 and young can be generated, that it seems unnecessary to look 

 further in order to account for their origin. But the precise 

 mode in which the countless brood of these parasites make their 

 way into the interior of the animals they are destined to inhabit, 

 was long but dimly understood, until by degrees attention was 

 directed to* certain peculiarities in the mode of life of these 

 creatures, which threw great light upon the subject. 



It has been ascertained, in fact, that at particular periods of their 

 existence, the intestinal worms undertake emigrations, and these 

 often very extensive ones, in order to reach that animal whose 

 organs are by nature fitted for their habitation. We now know 

 that the young of the tape-worm, (which inhabits the intestine of 

 the higher animals only,) leave the place where they were brought 



1 With respect to the tape-worms, it is well known that a single individual is often com- 

 posed of many hundred joints. Each joint is capable of laying many hundred ova, so 

 that the number of the progeny of a single tape-worm is enormous. Professor Eschricht 

 of Copenhagen (see his work ' Das Physische Leben in popularen Vortragen, Berlin, 

 1852, p. 115) posesses a tape-worm, expelled by a patient of his, which consists of 

 1000 joints, and some of the joints contain more than 1000 ova. The same writer 

 (ibid., p. 112) having carefully examined the reproductive organs of a female Ascaris 

 lumbricoides, estimates the number of eggs in a single thread-worm at many millions. 



