34 MANUAL OF ZOOLOGY. 



metamorphosis is rightly applied. These changes, however, do 

 not differ in kind from the changes undergone by a Mammal; 

 the difference being that in the case of a Mammal the ovum is 

 retained within the body of the parent, where it undergoes 

 the necessary developmental changes, so that at birth it has 

 little to do but grow, in order to be converted into the adult 

 animal. 



From these considerations we arrive at the second law laid 

 down by Quatrefages : " Those creatures whose ova owing to 

 an insufficient supply of nutritious contents, and an incapacity 

 on the part of the mother to provide for their complete de- 

 velopment within her own substance are rapidly hatched, 

 give birth to imperfect offspring, which, in proceeding to their 

 definitive characters, undergo several alterations in structure 

 and form, known as metamorphoses." 



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 Lernaese, both of which belong to 

 the Crustacea. 



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

 parasitic Lernaese 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 Cypris (Ostracoda). 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, repro- 

 ductive 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 



