ANIMAL AND PLANT REPRODUCTION 21 



attention, the end may be lost to sight through, admiration 

 of the ingenuity of the means. 



There seems to be no question but that sexual repro- 

 duction is a more recent means of propagation than asex- 

 ual reproduction. Although asexual reproduction in the 

 narrow sense, that is, by means of simple division or by 

 budding, is common among the 

 protozoa, the sponges, the coelente- 

 rates and the flat worms, it becomes 

 sporadic in the molluscoids and 

 annelids, and is found in only 

 one or two isolated instances in 

 forms as highly specialized as the 

 arthropods and the chordates. If 

 fragmentation succeeded by regen- 

 eration of the lost parts be conceded 

 to be a true means of reproduction, 

 however, echinoderms and nematode 

 worms are included. Thus of all 

 the great groups of animals only 

 certain worms (Trochelminthes) 

 and the molluscs have no asexual 

 reproduction in the usual sense of 

 the word, and zoologists would 

 hardly feel safe in maintaining its absence in these two 

 phyla since the life history of so many forms is unknown. 

 But since asexual reproduction is replaced by sexual 

 reproduction to a greater and greater extent as the higher 

 forms are reached one cannot avoid the conclusion that 

 the latter has proved to be the really successful means 

 of propagation. Nevertheless, variations appeared in 

 highly specialized forms which permitted return to an 



Fig. 1. — Asexual reproduc- 

 tion. An amceba in division. 

 cv. contractile vacuole; ek, ecto- 

 sarc; en, entosarc; n, nucleus. 

 (Kingsley after Schulze. Cour- 

 tesy Henry Holt & Co.). 



