OF THE GROWTH OF ANIMALS. 53 



lower ranks, and a more or less corresponding decrease in the de- 

 gree of dependence of the young upon the parent; for instance, 

 in the duck-billed quadruped, or Ornithorhynchus, (fig. 26,) the 

 young are born 

 while yet in a 

 far inferior state 

 of development to 

 that of the young 

 of the horse, or 

 cow, or dog, when 

 born. The young 

 of the kangaroos 

 and opossums, of which there are many species, are hardly more 

 advanced when born than the young Duck-mole. But there are 

 vertebrates which in a descending scale are for a less and less 

 time retained ; the eggs of birds are laid, that is, subjected to 

 the influence of surrounding external causes, at a time when 

 there is scarcely a trace within them of the so-called germ, which 

 from this period is altogether dependent upon one of the phys- 

 ical agencies, heat, for its development. Still lower in the scale 

 we find the eggs of frogs and toads, and some kinds of fishes, 

 are laid before they can hardly be said to have become fully 

 formed as eggs. And so we might go on, pointing out instances 

 of this decreasing degree of dependence, until we find the egg 

 expelled from the parent at such an early stage as to be dependent 

 upon physical agents during a large part of its period of growth ; 

 for instance, it is dependent upon temperature, or moisture, or 

 dryness, or light, or some form of physical help, exactly as are 

 spontaneously generated bodies, or as those developed in the 

 sealed flasks. 



Fig. 26. Ornithorliynchus paradoxus. Blum. Natural size, " as large as a 

 cat." The Duck-bill quadruped, or Duck-mole. Inhabits Australia. After 

 Bennet. 



