ALTERNATION OF GENERATIONS. 123 



metamorphosis, and hence the direct development, as opposed to 



the metamorphosis, is a secondary form of development. 



. 



ALTERNATION OF GENERATIONS, POLYMORPHISM AND HETEROGAMY. 



Both in direct development and indirect development by means 

 of a metamorphosis, the successive stages take place in the life- 

 history of the same individual. There are, however, instances of 

 free development, in which the individual only passes through a 

 part of the developmental changes, while the offspring produced by 

 it accomplishes the remaining part. In this case the life-history 

 of the species is represented by two or more generations of indivi- 

 duals, which possess different forms and organization, exist under 

 different conditions of life, and reproduce in different ways. 



Such a manner of development is known as alternation of genera- 

 tions (metagenesis), and consists of the regular alternation of a 

 sexually differentiated generation with one or more generations 

 reproducing asexually. This phenomenon was first discovered by 

 the poet Charnisso* in the Salpidae ; but the observation remained 

 for more than twenty years unnoticed. It was rediscovered by 

 J. Steenstrup, t and discussed in the reproduction of a series of animals 

 (Medusas, Trematoda) as a law of development. The essence of the 

 process consists in this, that the sexual animals produce offspring, 

 which through their whole life remain different from their parents, 

 but can give rise by an asexual process of reproduction to a gener- 

 ation of animals which resemble in their organization and habits 

 of life the sexual form, or again produce themselves asexually, their 

 offspring assuming the characters of the original sexual animal. 

 So that in the last case the life of the species is composed of three 

 different generations proceeding from one another, viz., sexual 

 form, first asexual form, and second asexual form. The development 

 of the two, three, or more generations may be direct, or may take 

 place by a more or less complicated metamorphosis ; similarly the 

 asexual and the sexual generations sometimes differ but little from 

 each other (e.g. Salpa), and sometimes present relations analogous 

 to those which exist between a larva and the adult animal (e,.<j. 



* Adalbert de Chamisso, " De animalibus quibusdam e classe vermium 

 Linnreana in circumnavigatione terras auspicante comite N. Eomanzoff duce 

 Ottone de Kotzebue armis 1815, 1816, 1817, 1818 pcracta." Fasc. I. De salpa 

 Berolini 1819. 



f Job. Jap. Sm. Steenstrup, " Ueber den Generationswechsel, etc," iibersetzt 

 von C. H. Loreuzen. Kopenhagen, 1842. 



