SPONTANEOUS GENERATION. 37 



Retrograde Development. Ordinarily speaking, the course 

 of development is an ascending one, and the adult is more 

 highly ^organised than the young ; but there are cases in which 

 there is an apparent reversal of this law, and the adult is 

 to all appearance a degraded form when compared with the 

 embryo. This phenomenon is known as "retrograde" or 

 "recurrent" development; and Well-marked instances are found 

 amongst the Cirripedia and Lernaeas, both of which belong to 

 the Crustacea. 



Thus, in the Cirripedes (acorn-shells, &c.) and in the 

 parasitic Lernaeae the embryo is free-swimming and provided 

 with organs of vision and sensation, being in most respects 

 similar to the permanent condition of certain other Crustacea, 

 such as the Copepods. The adult, however, in both cases, is 

 degraded into a more or less completely sedentary animal, 

 more or less entirely deprived of organs of sense, and leading 

 an almost vegetative life. As a compensation, reproductive 

 organs are developed in the adult, and it is in this respect 

 superior to the locomotive, but sexless, larva. 



12. SPONTANEOUS GENERATION. 



Spontaneous or Equivocal generation is the term applied 

 to the alleged production of living beings without the pre- 

 existence of germs of any kind, and therefore without the pre- 

 existence of parent organisms. The question is one which has 

 been long and closely disputed, and is far from being settled ; 

 so that it will be sufficient to indicate the facts upon which the 

 theory rests. 



If an animal or vegetable substance be soaked in hot or cold 

 water, so as to make an organic infusion, and if this infusion 

 be exposed for a sufficient length of time to the air, the follow- 

 ing series of changes is usually observed : 



1. At the end of a longer or shorter time, there forms upon 

 the surface of the infusion a thin scum, or pellicle, which, when 

 examined microscopically, is found to consist of an incalculable 

 number of extremely minute molecules. 



2. In the next stage these molecules appear, many of them, 

 to have increased in size by endogenous division, till they form 

 short staff-shaped filaments, called " bacteria." These increase 

 in length by the same process until we get long filamentous 

 bodies produced, which are termed " vibriones." "* Both the 



* By some authorities it is believed that the bacteria are produced by the 

 fusion together of the primitive molecules in twos and threes ; and that the 

 vibrios are produced out of the bacteria by the addition of fresh molecules 

 to the extremities of the latter, or by their uniting with one another. 



