5o8 POPULAR SCIENCE MONTHLY. 



their true relationsliip. Mollusks, like the oyster, have also been much 

 modified by fixation, and among insects the remarkable scale insects 

 present extreme results in this direction. It is not mere chance that 

 the oyster and the oyster shell bark louse have similar shape. They 

 have both been modified, quite independently and in different locations, 

 by the same controlling factors working on a sedentary organism. 

 ]\Iany other insects in one stage or another illustrate this phase, but 

 space forbids their mention. 



Tunicates have traveled this road and thereby lost the rich inheri- 

 tance that was theirs had they cultivated their backbone instead of 

 allowing it to pass into 'innocuous desuetude.' Even among verte- 

 brates we see some tendency to adopt the tied-up plan, for among the 

 fishes the lampreys attach themselves to other fishes, the remours to 

 the belly of the shark, the sea horse temporarily fastens to branches 

 of coral by wrapping around them his flexible tail, the flounder rests 

 almost fixedly at certain points, but throughout the group there is 

 practically no permanent fixity with the degeneration it entails. It 

 will be seen by those familiar with the forms cited that the mere fact 

 of an animal having become habitually attached in a certain place 

 and having lost its power of free movement has greatly affected its 

 structure and future possibilities. It has bettered its chances for sur- 

 vival, but it has sacrificed all hope of progressive development. I 

 believe it is quite safe to say that no high type of animal life can be 

 referred in its origin to a sedentary ancestry. 



Another frequent by-path is that of parasitism, and, indeed, so 

 common is this mode of life, so prevalent in some degree or other 

 among animals of almost every branch, that it would appear to be one 

 of the easiest roads to travel. But this road leads inevitably to restric- 

 tion of freedom and limitation of sphere — often to degeneration of 

 some portion of the organism. Its ultimate end is extreme limitation 

 and probable extinction. Protozoans, coelenterates, worms in great 

 numbers, crustaceans, insects, mollusks and even some remarkable ver- 

 tebrates have followed this road, and in every case where the habit has 

 gone to any great extent it would seem impossible for them to retrace 

 the route. Wherever the parasite has become limited to a single host 

 or to alternate hosts, destruction of the host form means death to the 

 parasite, and extinction of the host would mean extinction of the 

 parasite. Loss of wings in formerly winged forms, loss of eyes and 

 other organs of sense, loss of nervous system, loss of motion, loss of 

 digestive organ even, in extreme cases, are the penalty they pay. 

 ' Sans eyes, sans ears, sans nose, sans mouth, sans everything, ' but actual 

 necessities of existence and reproduction. 



At first thought it may not seem so strange that wastes of desert 

 land should present no small degree of living activity, but if we notice 



