490 LIFE HISTORY STUDIES OF ANIMALS. 



her owu body. The young will in such cases complete their devel- 

 opment as embryos, and when hatched will resemble the parent in 

 everything but size. 



Which policy is adopted will largely depend nyow the number of the 

 family and the capital at command. There are animals which are like 

 well to-do people, who provide their children with food, clothes, school- 

 ing, and pocket money. Their fortunate offspring grow at ease and 

 are not driven to premature exercise of their limbs or wits. Others 

 are like starving families, which send the children, long before their 

 growth is completed, to hawk matches or newsi)apers in the streets. 



In biology we have no sooner laid down a principle than we begin to 

 think of exceptions. The exceptions may be apparent only; they 

 may, when fully understood, confirm instead of disturbing the general 

 principle. But this rarely happens unless the principle is a sound one. 

 Exceptio probat regulam ; it is the exception which tests the rule, to 

 give a new application to an old maxim. 



Parasites form one group of exceptions to our rule. Whether they 

 pass their free stages in air, water, or earth, whether their hosts are 

 marine, fluviatile, or terrestrial, they are subject to strange transforma- 

 tions, which may be repeated several times in the same life history. 

 The change from one host to another is often a crisis of difficulty. 

 Many fail to accomplish it. Those which succeed do so by means of 

 some highly peculiar organ or instinct, which may be dropped as 

 quickly as it is assumed. The chances of failure often preponderate 

 to such an extent that an enormous number of eggs must be liberated. 

 Even a brief parasitism may produce a visible effect upon the life 

 history. The young Unio or Anodon attaches itself for a short time to 

 some fish or tadpole. To this temporary parasitism is due, as I sup- 

 pose, the great number of eggs produced, and a degree of metamor- 

 phosis unusual in a fresh-water mollusk. 



The cephalopoda, which are wholly marine, and the vertebrates, 

 whatever their habitat, very rarely exhibit anything which can be 

 called transformation. Some few cases of vertebrate transformation 

 will be discussed later. Cephalopods and vertebrates are large, strong, 

 quick-witted animals, able to move fast, and quite equal in many cases 

 to the defense of themselves and their families. They often produce 

 few young at a time and take care of them (there are many examples 

 to the contrary among cephalopods and fishes). They are generally 

 able to dispense with armor, which would have indirectly favored 

 transformation. 



Echiuoderms which are all marine, develop with metamorphosis. 

 There is an interesting exception in the echiuoderms with marsupial 

 development, which develop directly and give an excellent illustration 

 of the effect of parental care. 



Insects, which as terrestrial animals should lay a few large eggs, 

 and develop directly, furnish the most familiar and striking of all 



