274 



Nature of 

 metamor- 

 phosis. 



How lost. 



HABIT AND INTELLIGENCE. 



[chap. 



Metamorphosis is due to variations taking place not at 

 an early age, and being inherited at a corresponding age in 

 the offspring.^ 



Metamorphosis, or metagenesis, may be lost by the cha- 

 racters of the mature form appearing at birth instead of 

 some time after it. This is a very probable result of 

 spontaneous variation ; but the opposite change appears 

 impossible. That is to say, it is a very probable variation 

 for a species that has habitually undergone a metamor- 

 phosis, to acquire its mature form by direct development, 

 and so to lose its metamorphosis ; but it appears impossible 

 that any species which has habitually acquired its mature 

 form by direct development, should, as a result of any 

 variation, begin to appear first in a larval form. I deduce 

 this from the law of habit, that an inherited character 

 sometimes appears in the offspring at the same age as that 

 at which it appeared in the parent ; sometimes earlier, but 

 seldom or never later. 



If any one, previously unacquainted with the subject, 

 considers and compares the facts that have been brought 

 together in this chapter, he may very probably make some 

 comment like this : " A plausible explanation is here 



1 Tliis does not apply to metagenesis. But metagenesis, though a less 

 familiar fact than metamorphosis, is not nearly so difficult to account for. 

 Such metagenesis as that of Aphis or of Cecidomyia (a dipterous insect) con- 

 sists in the generation of larvje by the larva, and needs no special explana- 

 tion : what does need explanation is the metamorphosis of the larva into 

 the perfect form, which, in Aphis at least, always occurs at intervals of a 

 few generations. Such metagenesis as that of the Hydrozoa consists in 

 the reproductive organs becoming detached, which may take place as the 

 result of a very slight variation. 



In metamorphosis, form A is transformed into form B (as the caterpillar 

 into the butterfly), and form B produces form A again. In metagenesis, 

 form A 2rroduccs foi-m B, and form B produces form A again, as in the case 

 of the hydra-like Hydrozoon and its Medusa. In Aphis, the two pro- 

 cesses are complicated together. Larvas produce larvae for several genera- 

 tions, and perish without imdergoing metamorphosis; but at intervals 

 they are metamorphosed into the winged state, and a new generation of 

 larvae is produced from their eggs. This is classed as a case of metagenesis, 

 because there is one form in the cycle that remains permanently unlike 

 the other : the larva that dies without undergoing metamorphosis never 

 becomes like the winged form. 



