THE LA W OF LIKENESS, AND ITS WORKIXG. 209 



The great Harvey, whose researches on animal develop- 

 ment may be regarded as having laid the foundation of 

 modern ideas regarding that process, adopted as his physio- 

 logical motto the expression, Omne animal ex ovo. Whilst 

 it is undoubtedly true that the egg, or ovum, must be re- 

 garded as the essential beginning and type of development 

 in animals, we note that, as in nai's, the production of new 

 beings is not solely dependent on the presence of that 

 structure. Just as plants are propagated by slips and cut- 

 tings, so animals may be developed from shoots or specially 

 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 denned stages of that 

 process, that the exceptions to the law of likeness are 

 most frequently met with. 



One of the most remarkable deviations from the normal 

 law of development is seen in the case of the little Aphides 



\ I. Plant-lice, natural size (a, b] and magnified : A, wingless, and B, winged form. 



(Fig. 28) or plant-lice, the insects so familiar to all as 

 the pests of the gardener. At the close of the autumn 

 season, winged males and females of these insects appear 

 amongst their neighbour-aphides, and these produce eggs, 

 which, however, lie dormant throughout the winter. Waking 

 into life and development with the returning spring, these 

 eggs give birth each to a wingless female ; no insect 

 of the sterner sex being found amongst the developed 

 progeny of these insects. The presence of both sexes is 



p 



