208 THE SEVEN FOLLIES OF SCIENCE 



which the larvae were developed simply because he did 

 not look for them. Consequently it was the most natural 

 thing in the world for him to suppose that they owed 

 their origin to the putrefying action of the carcass itself. 

 As we examine other forms of animal life the difficulty of 

 ascertaining their origin becomes in many cases very great 

 and in the case of some parasites it has required the labo- 

 rious efforts of the ablest biologists to make out their life 

 history. Until a few decades ago the different life stages 

 of certain marine animals were regarded as entirely differ- 

 ent species and each stage was classified as being an en- 

 tirely distinct animal. And it is within the memory of 

 living men that the parr, a small fish which swarms in all 

 salmon rivers, was considered a distinct species and was 

 allowed to be slaughtered without limit, whereas it is now 

 known beyond the possibility of a doubt that it is the 

 young of the salmon and is carefully protected. 



And it is now very certain that no creatures which show 

 distinct animal characteristics ever appear except as the 

 progeny of animals of the same kind. 



THAT SMALL FLIES ARE THE YOUNG OF 

 LARGE FLIES 



VERY observing person must have noticed that 

 the flies which infest our houses differ greatly in 

 size, some being very small while others of simi- 

 lar general appearance are quite large. And it 

 is a very common idea that these small flies are small 

 simply because they are young and that if they are allowed 

 to live they will grow larger. It is very natural that this 



