THE LA W OF LIKENESS, AND ITS WORKING. 



553 



body — represented by the front segment. A 

 new head, eyes, and appendages, could be traced 

 in course of formation upon the front extremity 

 of each of the new segments ; and, as development 

 terminated, each portion could be seen to gradu- 

 ally detach itself from its neighbors ; the original 

 worm thus resolving itself into four new individ- 

 uals. The most curious feature regarding this 

 method of development consists in the fact that 

 the bodies of these worms and of nearly-related 

 animals grow by new joints being added between 

 the originally-formed segments and the tail. If, 

 therefore, we suppose that one of these new joints 

 occasionally develops into a head, we can form 

 an idea of the manner in which a process, origi- 

 nally intended to increase the growth of one and 

 a single worm, becomes competent to evolve new 

 individuals, each of which essentially resembles 

 the parent in all particulars. 



The great Harvey, whose researches on ani- 

 mal development may be regarded as having 

 laid the foundation of modern ideas regarding 

 that process, adopted as his physiological motto 

 the expression, omne animal ex ovo. While it is 

 undoubtedly true that the egg, or ovum, must be 

 regarded as the essential beginning and type of 

 development in animals, we note that, as in Na'is, 

 the production of new beings is not solely de- 

 pendent on the presence of that structure. Just 

 as plants are propagated by slips and cuttings, 

 so animals may be developed from shoots or spe- 

 cially detached portions of the parent-body. And 

 it is in the development of the egg, or in the 

 course of what may be regarded as the most 

 regular and defined stages of that process, that 

 the exceptions to the law of likeness are most 

 frequently met with. One of the most remark- 

 able deviations from the normal law of devel- 

 opment is seen in the case of the little aphides, 

 or plant-lice, the insect so familiar to all as the 

 pests of the gardener. At the close of the au- 

 tumn season, winged males and females of these 

 insects appear among their "neighbor aphides, 

 and these produce eggs, which, however, lie dor- 

 mant throughout the winter. Waking into life 

 and development with the returning spring, these 

 eggs give birth each to a wingless female ; no in- 

 sect of the sterner sex being found among the 

 developed progeny of these insects. The pres- 

 ence of both sexes is throughout the animal 

 world regarded as necessary for the production 

 of eggs capable of developing into offspring. 

 Strangely enough, however, these wingless fe- 

 males not only produce eggs, hatching them 

 within their bodies, but the eggs develop into 



beings exactly resembling themselves, not a sin- 

 gle male aphis being represented within the 

 limits of this Amazonian population. Seven, 

 eight, nine, or even eleven generations of these 

 wingless females may be produced in this man- 

 ner, and the swarms of plant-lice which infest 

 our vegetation attest the fertility of the race. 

 But in the last brood of these insects, produced 

 toward the close of autumn, winged males appear 

 in addition to the females, which latter also pos- 

 sess wings. The members of this last brood 

 produce eggs of ordinary nature, which lie dor- 

 mant during the winter, but which in the suc- 

 ceeding spring will inaugurate the same strange 

 life - history through which their progenitors 

 passed. The case of the plant-lice may for the 

 present be dismissed with the observation that 

 the law of heredity appears to operate in this 

 instance in a somewhat abnormal, or, at any 

 rate, in a very unusual manner. The true simili- 

 tude of the winged parents is not attained until 

 after the lapse of months, and through the inter- 

 ference, as it were, of many generations of dis- 

 similar individuals ; while no less worthy of re- 

 mark is the circumstance that one sex alone is 

 capable of giving origin to new beings, which 

 sooner or later produce in turn the natural dual- 

 ity of sex, forming the rule of both animal and 

 plant creation. And the case of the plant -lice is 

 rendered the more remarkable by the considera- 

 tion that of 58,000 eggs laid by female silk-moths 

 which were separated from the opposite sex, only 

 twenty-nine developed into perfect caterpillars — 

 the female plant-lice possessing a fertility under 

 like circumstances which would be amazing even 

 if taking place under the normal laws and con- 

 ditions of development. 



Cases of the unusual development of ani- 

 mals, which serve as parallel instances to the 

 case of the plant-lice, are by no means rare. 

 Thus in the case of the starfishes, sea-urchins, 

 and their neighbors, the egg gives origin to a 

 free-swimming, active body, which develops a 

 structure of its own, and appears in a fair way 

 to become, as might bo expected, the future 

 starfish. But within the body of this first em- 

 bryo another formation is seen to take place ; 

 and sooner or later this secondary development 

 comes to assume priority, and appears as the 

 true and veritable representative of the young 

 starfish — the primitive body or embryo which 

 produced it being either absorbed into its sub- 

 stance, or cast off on development being fully 

 attained and completed. The production of the 

 second starfish, as it were, out of a first-formed 



