672 REPORT— 1897. 



and pei-liaps watch over tliem, or even hatch thero within her own hody. The 

 young will in such cases complete their development as embryos, and when 

 hatched, will resemhle the parent in everything but size. 



Which policy is adopted will largely depend upon 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, schooling, 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 newspapers 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 under- 

 stood, confirm instead of disturbing the general principle. But this rarely 

 happens unless the principle is a sound one. Exwptio probat regzdam ; 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 transformations, 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 

 iDy 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 para- 

 sitism 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 suppose, the great number of eggs produced, and a degree of 

 metamorphosis, unusual in a fresh-water moUusk. 



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 defence 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 armour, which would have indirectly I'avoured trans- 

 formation. 



Echinoderms, which are all marine, develop with metamorphosis. There is 

 an interesting exception in the Echinoderms with marsupial development, which 

 develop directly, and give an excellent ilhistration of the efi'ect 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 transformations. I have 

 already discussed this case at greater length than is possible just now.* I have 

 pointed out that the less specialised insect-larvfe, e.g. those of Orthoptera, make a 

 close approach to some wingless adult insects, such as the Thysanura, as well as 

 to certain Myriopods. Fritz Miiller seems to me to be right in saying that the 

 larvae of non-metamorphic insects come nearer than any winged insect to primi- 

 tive Tracheates. The transformation of the Bee, Moth, or Blow-fly is transacted 

 after the stage in which the normal Tracheate structure is attained, and I look 

 upon it as a peculiar adzdt transformation, having little in common with the 

 transformations of Echinoderms, Mollusks, or Crustaceans. 



In the same way I believe that some Amphibia have acquired an adrilt trans- 

 formation. Frogs and toads, having already as tadpoles attained the full develop- 

 ment of the more primitive Amphibia, change to lung-breathing, tailless, 

 land-traversing animals, able to wander from the place of their birth, to seek 

 out mates from other families, and to lay eggs in new sites. 



IMedusre furnish a third example of adult transformation, which seems to find 

 its explanation in the sedentary habit of the polyp, which probably nearly 

 approaches the primitive adult stage. But here the case is further complicated, 



' Nature, Dec. 19, 1895. 



