IX TROD UCTION. aa 



Amongst Poultry we have at least one serious < vege- 

 table* malady— the so-called ' Favus,' disfiguring the 

 comb, wattles, and head generally of the bird. The 

 minute ' spores ' or seeds of these vegetable parasites 

 are carried about in the air, so that they have a chance 

 in the ' struggle for life ' of increasing their race by 

 other means than direct contagion. 



It must be remembered, however, that these para- 

 sitic diseases are highly infectious. Many of the 

 parasitic fungi have most interesting and almost 

 marvellous life-histories ; but as this class of disease 

 is seldom manifest in the Fowl, they will only be 

 brief! v referred to in this small Manual. If vegetable 

 parasites are wonderful, we must expect to find similar 

 extraordinary cases of this degraded form of life in 

 animals. In the animal world nearly every group of 

 invertebrates has its degenerate members, some so far 

 debased by their methods of living as to become almost 

 unrecognizable in regard to their true relationship. 

 With parasitism we find concomitant the loss of 

 many structural elements owing to disuse : for 

 instance, some of the small Crustacea that live 

 entirely in fish, and which have no need of legs 

 or even organs of sense, so needful in the struggle 

 for life, have entirely lost these organs and append- 

 ages. On the other hand, there are many Entozoa 

 that still retain their typical form. The more 



