Page:Popular Science Monthly Volume 63.djvu/512

508 their true relationship. 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. Many 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 inheritance that was theirs had they cultivated their backbone instead of allowing it to pass into 'innocuous desuetude.' Even among vertebrates 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 survival, 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 restriction 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 vertebrates 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