DISTRIBUTION OF REPRODUCTIVE METHODS 265 



the larger cells lost their swimming flagella and were brought 

 into contact with the smaller cells only by the motions of 

 the latter, which still retained their flagella. Lastly, most 

 of the cells of an organism ceased to have any share in 

 reproduction, being simply concerned in the life of the colony. 

 Some of the cells in such a colony, however, assumed as their 

 part the process of uniting with others, and thus carried on 

 the functions of reproduction. These cells still continued to 

 differentiate into large and small cells, the large ones becoming 

 eggs and the small ones remaining as sperms. From this 

 time on the function of reproduction is independent of the 

 functions of the life of the colony, and the individual exists 

 apart from its offspring. From all of this it appears that con- 

 jugation is the first step in the direction of sex union, and 

 that conjugation must therefore be regarded as a form of sex 

 union, although the sexes have not been sharply differentiated 

 in any true case of conjugation. 



DISTRIBUTION OF ASEXUAL REPRODUCTION 



Among plants asexual reproduction is nearly universal, 

 all of the lower plants, and nearly all the higher ones, being 

 able to multiply by some form of budding or division. Par- 

 thenogenesis is also fairly frequent. Among animals multi- 

 plication by budding or division is also widely distributed. 

 It is universal among the unicellular animals, and is a common 

 method of multiplication among such lower forms as Hydra 

 and its allies. As we pass to higher animals this power dis- 

 appears. It is found among some worms, and one group of 

 animals related to the vertebrates (Tunicata) forms colonies 

 by budding, which may break up and become several colonies, 

 this constituting a modified kind of reproduction. In no other 

 higher animals does asexual reproduction occur. The modified 

 type of asexual reproduction which is called parthenogenesis 

 is found among some of the higher animals, being fairly com- 

 mon even among insects. 



