40 THE PHYSICAL KINSHIP 



not much similarity between a water-strider and 

 a butterfly or between a stag-beetle and a gnat. 

 But they are all, in reality, built according to the 

 same plan. Like all other insects, they have six 

 legs, a sheath-like skeleton, and bodies character- 

 istically divided into head, thorax, and abdomen. 

 It is the same with all other great classes of beings. 

 All worms resemble each other ; and so do all 

 mollusks, although they may differ in particulars 

 as widely as nautiluses and clams. Echinoderms 

 have a radiate structure, celenterates and sponges 

 are vase-like in shape, and protozoa are one-celled. 

 The differences in structure among the members 

 of a group consist in different modifications of a 

 fundamental type. Among the vertebrates the 

 fore-limb may be an arm, a leg, a wing, a shovel, a 

 flipper, or a fin. But in all cases it is the same 

 organ — that is, the same implement modified to 

 serve different ends. Take the mouth-parts of 

 insects. In the grasshopper and cricket these 

 parts are fitted for grinding; in the moths and 

 butterflies they are fashioned into long tubes for 

 sucking the sweets of flowers; in the mosquito they 

 form an elaborate apparatus for drilling and drink- 

 ing ; and in the mayfly the mouth-parts, though 

 present, are not used at all. In all of these animals 

 these parts are essentially the same, although differ- 

 ing so much in their forms and purposes that the 

 unscientific can scarcely be made to believe they 

 are fundamentally alike. There is no fact more 

 familiar to the biologist or more frequently met 

 with in the fields of animal morphology than the 



