154 



NA TURE 



[December 19, 1895 



(among others) to fulfil. It must feed and grow ; it must 

 also separate from its fellows, and find out an unoccupied 

 site. The two functions may be discharged together. 

 Birds, for instance, feed all round the year, and change 

 their abode whenever they suffer from overcrowding or 

 scarcity of food. In other cases, either function may for 

 special reasons be limited to a particular time of life. 

 The crustacean cannot migrate effectively when adult, 

 because of the heavy armour which it carries. The 

 insect cannot migrate effectively when young, because 

 of the difficulty of travel by land. It is easy for'a minute 

 animal to travel by sea. The high specific gravity of 

 the water renders the body buoyant, and currents, even 

 where special locomotive organs are absent, will do the 

 work of transport. Power to rise and sink in the water 

 is convenient, but even this is not strictly indispensable. 

 On land the conditions are very different. The weight 

 of the body has to be supported in a comparatively rare 

 ■ medium, and much greater exertion is called for Run- 

 ning, leaping, and flying are difficult exercises, much 

 better performed by the adult than by the larva, if these 

 should happen to differ materially. 



It is often of advantage that there should be division 

 of labour between the several stages of the life-history, 

 and the functions of migration and growth may be 

 allotted to special times of life instead of being carried 

 on throughout. Migration is naturally associated with 

 lightness and activity ; feeding and rapid growth are 

 favoured by a sedentary habit. If such division of 

 labour should take place, as it commonly does in crus- 

 taceans and insects, the crustacean will find it convenient 

 to perform its migration early, when it has little armour 

 to carry, and when its surface is large in proportion to 

 its weight. The Zoa;a will do little more by its own 

 exertions than maintain the right attitude and the right 

 distance from the surface of the sea, leaving the currents 

 to effect the actual transport from place to place. Migra- 

 tion over, it will settle to the bottom, acquire heavy 

 armour as a defence from its many enemies, and begin 

 to feed in earnest, becoming in the end a slow and heavy 

 inhabitant of the sea-bottom. With the insect the pro- 

 cedure will be reversed. Being unable to travel far while 

 small and weak, it will feed first, and having attained 

 its full size, will then, if at all, acquire special means of 

 locomotion. Wings are more efficient as a means of 

 transport than any other organs of locomotion of which 

 a terrestrial animal can avail itself; but flight is so 

 difficult an exercise that wings, if acquired at all, will 

 be acquired late. 



Marine animals commonly produce far more eggs than 

 insects.^ The risks of the shallow seas are so great 

 that a small proportion only of the young animals comes 

 to rnaturity. Hence the enormous fertility of common 

 marine animals, except such as are able to nourish or 

 defend their young. Where a vast multitude of eggs 

 hatch out together, dispersal in search of food becomes 

 an immediate necessity. 



The more sluggish and sedentary the adult, the greater 

 the activity we may expect to find in the larvae. It is 

 they which have to travel and to find out suitable 

 quarters. But they often make up by their numbers for 

 any deficiency in enterprise or inteUigence. Vast numbers 

 of Zoceas are swept into mid-ocean or into tidal rivers, 

 or are devoured. It is only a chance remnant that 

 survives. 



Just as the sluggishness of the adult crustacean or 

 echinoderm may promote activity in the young, so the 

 activity of the adult insect may discourage activity in 

 the young. The power of flight possessed by most adult 

 insects favours a sedentary life in the larva, which is 



1 No statistics h.-ive been collected to my knowledge on an extensive 

 scale. Very many insects lay their eggs singly. In particular cases (social 

 insects, Meloidae, &c.), where special reasons obtain, they may be laid in 

 great numbers, but my general impression is that the eggs of Echinoderms 

 ■or marine Crustaceans would be far more numerous as a rule. 



NO. 1364. VOL. 53] 



spared all effort in connection with the dispersal of the 

 species, and can give its undivided attention to feeding. 

 Accordingly the larva often degenerates more or less, 

 especially when its food is abundant and obtained with- 

 out labour or risk. The sagacity and industry of the 

 bee in storing its cells with honey may be said to have 

 cost the larva its legs. So, too, the active and highly- 

 gifted blow-fly, ichneumon, and gall-fly secure abundant 

 food for the new generations, which emerge as footless 

 grubs. In the Coleoptera we find that the larvse, which 

 feed on dead vegetable matter beneath the surface of the 

 ground, are heavy-bodied and short-legged, sometimes 

 so overloaded by the weight of the abdomen that they 

 rest lying on one side. The larva; of weevils, which are 

 vegetable-feeders and often buried in the plant which 

 they devour, are commonly without legs altogether. Even 

 the head of the larva may be reduced almost to nothing 

 by the labours of the adult insect. The blowfly finds 

 out a practically inexhaustible supply of carrion in which 

 to lay its eggs, and the larval head is so reduced that 

 it consists mainly of a pair of large hooks, with muscles 

 to work them, and a chitinous frame to form a fulcrum 

 for the muscles. 



The degeneration of the larva can only proceed far 

 when the adult undertakes the dispersal of the species. 

 That is why, it seems to me, the larvae of marine animals, 

 though often very unlike their parents, are not really to 

 be called degenerate. They cannot be supposed to have 

 arisen from ordinary forms, typical of the class, by mere 

 disuse and reduction of the organs of active life. On the 

 contrary, they are often immature forms specialised for 

 locomotion. 



Even when the adult undertakes the dispersal of the 

 species, the larva does not necessarily become de- 

 generate. The winged locust is the chief agent in 

 dispersal, but the larvae are active, seek their own food, 

 and exhibit no marks of degeneration. It is only when 

 their choice and responsibility are taken away, when 

 they are encouraged to feed almost without intermission, 

 or to bury themselves out of sight, that they degenerate. 



Parental care and all labours undertaken by the parent 

 for the sake of the offspring tend to promote helplessness 

 in the young. The helplessness of the higher Vertebrates 

 at birth, as well as their prolonged foetal development, 

 are due to the fact that their parents are able to find them 

 food and protection. Here there is no degeneration in 

 the young ; though helpless, they are of the same 

 zoological grade as their parents. The locomotive 

 organs and the senses of a baby or a nestling bird are as 

 complex as those of the adult, and are merely feeble for 

 want of skill and exercise. In the larval insect there 

 may be real degeneration, fewer limbs, fewer joints in 

 the limbs, deficiency of sense-organs, relative preponder- 

 ance of the organs of nutrition. If the transformation of 

 insects had never been traced, the structure of the two 

 stages would have inevitably led zoologists to place the 

 caterpillar in a lower class or order than the butterfly. 



There has been much speculation as to the form of 

 primitive insects. Fritz Miiller supposes that the wing- 

 less Orthoptera come near to the original stock, while 

 others have derived all insects from the wingless and 

 non-metamorphic Thysanura. By taking what is common 

 to the least modified Orthoptera and the Thysanura, and 

 rejecting all features peculiar to either, we shall certainly 

 get a highly generalised insect, such as might possibly 

 be a common ancestor for the whole group. Brauer 

 pointed out that such primitive insects still survive in 

 Campodea and Japyx, which have mouth-parts (as 

 Lubbock considers) "intermediate between the man- 

 dibulate and haustellate types," one pair of many-jointed 

 antennae, three pairs of long legs, and an abdomen 

 whose first seven segments bear pairs of rudimentary 

 appendages, while the tenth and last segment bears in 

 Campodea a pair of many-jointed appendages, in Japyx 



