xlviii THE ANIMAL KINGDOM. 



in all discussions the greatest care must be taken to discriminate between the ancestral 

 and the adaptive features. We can best illustrate this by taking the case of the de- 

 velopment of a mammal, and showing how in its various stages it presents a compen- 

 dium of its history, When, in the mammalian germ, the nervous system and 

 notochord arise, it is on a level with the larva of an Ascidian ; with the formation of 

 protovertebi-ae, it represents the Amphioxus ; a brain, and gill clefts and limbs, indicate 

 a fish and amphibian stage ; the development of an allantois and closure of the gill 

 clefts places it on an avian plane ; while with the appearance of a placenta the mam- 

 malian features are assumed. These successive stages of the individual are closely 

 paralleled by that of the class. The fleshy, boneless form of Amphioxus and the 

 tunicates would not be preserved, but from fishes to man the sequence of remains in 

 the rocks accords with that derived from embryology. It must not be understood 

 from this that the mammals have been derived from the birds. The true line of descent 

 is far different, as will be explained on a subsequent page. It merely indicates that 

 the mammal and the bird have arisen from a common stock, and have pursued the 

 same course during a portion of their history. 



Alteenation of Generations and Parthenogenesis. 



Having spoken of the normal method of development of animals, we may turn to 

 certain unusual or abnormal modes of production. As an example of what is known 

 as alternation of generations may be cited the history of the jelly-fish, such as the 

 naked-eyed medusae (Melicertum and Campanularia), which at one time of life 

 develop by budding, at another by eggs ; of the trematode worms, the adult forms of 

 which lay eggs, while the redia or proscolex of the same worm produces cercariiB by 

 internal budding. Here also may be cited the cases of strobilation of Aurelia, the 

 tape-worm, JVais, Syllis, and Autolytus, among annelids. Thus among coelenterates 

 and worms, as well as some Crustacea, a large number of individuals are produced, 

 not from eggs, but by budding. 



Similar occurrences take place among insects, as the Aphis or plant-louse, in which 

 a virgin Aphis may bring forth in one season nine or ten generations of Aphides, so 

 that one Aphis may become the parent of millions of young. These young directly 

 develop from eggs or buds which are never fertilized, hence the term parthenogenesis, 

 or virgin-reproduction, sometimes called agamogenesis (or birth without marriage). 

 The bark-lice as well as the Aphides develop in this manner during the warm weather; 

 but at the approach of cold both male and female Aphides and Coccidae appear, the 

 females laying fertilized eggs, the first spring brood thus being produced in the nor- 

 mal, usual manner. 



Still more like the production of young in the redia of the trematode worms is the 

 case of the larva of a small gall-gnat (Miastor), which during the colder part of 

 the year from autumn to spring produces a series of successive generations of larvae 

 like itself, until in June the last brood develops into sexually mature flies, which lay 

 fertilized eggs. 



While the larval Miastor produces young like itself, the pupa of another fly, 

 Chironomus, also lays unfertilized eggs from which the flies arise. 



A number of moths, including the silk-worm moth, are known to lay unfertilized 

 eggs which produce caterpillars. Among the Hymenoptera, the currant saw-fly, cer- 

 tain gall-flies, several species of ants, wasps (Polisies), and the honey-bee, are known 

 to produce fertile young from unfertilized eggs ; in the case of the ants and bees, the 



