DEVELOPMENT. 199 



driven to premature exercise of their limbs or wits. Others are 

 like starving families, which are forced to send their children 

 to sell matches or newspapers in the streets. It is a question of 

 the amount of capital or accumulated food which is at command. 



The connection between zoological rank and the absence of 

 metamorphosis is also explained by what we see among men. 

 High zoological position ordinarily implies strength or intelli- 

 gence, and the strong and knowing can do better for their 

 offspring than the puny and sluggish. It does not cost a Shark 

 or a quadruped too much to hatch its young in its own body, 

 while Spiders and Earwigs,* which are among the highest 

 Invertebrates, defend their progeny, as do Mammals and Birds, 

 the highest Vertebrates. 



But what has all this to do with habitat ? Are fluviatile and 

 terrestrial animals, as a rule, better off than marine animals ? 

 Possibly they are. In the confined and isolated fresh waters at 



/ V 



least, the struggle for existence is undoubtedly less severe than 



O O u 



* It may be useful to point out the following examples of parental care among 

 animals in which, as a rule, the eggs are left to take care of themselves. It will be 

 found that in general this instinct is associated with high zoological rank (best 

 exemplified by Mammals and Birds), land or freshwater habitat, reduced number of 

 eggs, and direct development. 



AMPHIBIA. The eggs are sometimes hatched by the male (Alytes obsletr leans, 

 Rhinoderma Dancinii), or placed by the male in pouches on the back of the 

 female (Ptpa dorsiyera, Notodelphis ovifera, Nototrema marsupiatum), or 

 carried during hatching by the female (Polypedates reticulatus) . 



FISHES. The Stickleback and others build nests. Of eleven genera of nest- 

 building Fishes, eight are freshwater. The number of eggs is unusually small. 

 Many Siluroids have the eggs hatched in the mouth of the males, a few under 

 the belly of the female. The species are both marine and freshwater, the eggs 

 few and large. Lophobranchiate fishes usually have the eggs hatched by the male. 

 They are marine ; the eggs few and large. Many sharks hatch their eggs, which 

 are very few, within the body. Mustelus Icevis has a placenta, formed out of the 

 yolk-sac. 



INSECTS. De Geer has described the incubation of the Earwig, and the care of 

 the brood by the female. The cases of the social Hymenoptera, &c., are 

 universally known. 



SPIDERS. The care of the female spider for her eggs is well known. 



CBUSTACEANS. The Crayfish hatches and subsequently protects her young. Mysis, 

 Diastylis (Cuma), and some Isopods hatch their eggs. Gammarus locusta is 

 followed about by her brood, which shelter beneath her when alarmed. Podocerus 

 capillatus builds a nest among corallines. Several of the CapreUidce hatch or 

 otherwise protect their young. All these, except the Crayfish, are marine ; the 

 eggs commonly fewer than usual. 



ECHINODERMS. Many cases of "marsupial development" have been recorded 

 in the species of the Southern seas. Here development, conti ary to the rule in 

 Echinodermata, is direct. 



