NATURAL HISTORY. 



the ancestor of the form had those peculiarities in its adult condition, and 

 comparing the embrvos of various animals he can perceive their rela- 

 tionship one°to another, and can say at once which form is higher and 

 whirl, lower in the systematic scale. At the same time, in drawing his 

 inferences, he has to weigh his facts and ascertain what of the features 

 before him are of ancestral value, and what are adaptive; what are 

 derived from the adult, progenitor, and what have been acquired during 

 the larval stages. Processes of this sort call for reasoning powers of the 

 highesi class. In mathematics one is forced to just such and such con- 

 clusions—he has no alternatives; but in natural history other elements 

 enter in and must be weighed and estimated before valid conclusions can 

 drawn ; and besides, there is always an unknown quantity and a con- 

 siderable personal equation to be allowed for. 



There are several well-marked groups of arthropods, the relationships 

 of which are not yet settled, but for our purposes we may group them all 

 in live great divisions, typified by (1), the crabs and shrimps ; (2), the 

 .spiders; (3), the centipedes; (4), the six-footed forms like beetles and flies 

 md butterflies; and, lastly, a peculiar form, Peripatus by name, the position 

 of which is far from certain. 



