258 BIOLOGY OF THE PROTOZOA 



latter lives until a corresponding age, and dies in turn. The more 

 than one hundred and forty series that have followed one another 

 since 1917, in the same medium and under the same conditions, in 

 the same rhythmical cycles and with surprising uniformity, furnish 

 strong evidence that the environmental conditions have been suit- 

 able or " normal." For each series there has been the same sequence 

 of physiological conditions— high vitality and sexual immaturity, 

 encystment power, sexual maturity, decline in vigor and ultimate 

 death. If these phases of vitality are normal, if encystment and 

 reorganization, and conjugation are normal phenomena in the life 

 history of a ciliate then the conditions under which they occur must 

 likewise be normal. A hypercritical mind may deny the existence 

 of conjugation in Nature and maintain that conjugation occurs 

 only under the abnormal conditions introduced when the samples 

 are collected and transferred to small holders in the laboratory. 

 With such an individual convincing proof is apparently impossible 

 and we can only ignore the implication that conjugation is a phe- 

 nomenon which did not occur under " normal " conditions in Nature 

 but manifested itself only when man began to collect material. 

 I have no sympathy with such a point of view; I regard conjugation 

 as an entirely "normal" process in ciliates as gamete formation and 

 fertilization are "normal" processes in Sporozoa and Sarcodina. 

 When the conditions of the environment are such that this phe- 

 nomenon does not occur, then we may justly look for the unusual 

 at least. In a similar connection M. Robertson (1929) states: 

 " As the outcome of all the experimental work discussed above, the 

 American workers (i. e., the Woodruff school) deny the existence 

 of a life cycle in ciliates. To the present writer this seems an 

 erroneous attitude. . . . The result of this series of investiga- 

 tions is to show that the cycle is not a rigidly internally conditioned 

 sequence but is the response of an internally adaptable organism 

 to the external stimulus of the environment" (p. 163). The limits 

 of adaptation of protoplasm are unknown to us; it is quite con- 

 ceivable that conditions may be so arranged that for long periods 

 the normal sequence of phenomena in a life cycle are in abeyance 

 and the impression is gained that protoplasm under such conditions 

 has the possibility of indefinitely continued existence. But can this 

 be considered a normal environment? Here the conditions which 

 lead to conjugation are not offered and such conditions, if any, 

 might reasonably be regarded as abnormal; if conjugation is needed 

 the need is met by the artificial conditions and the organism is more 

 or less adapted to them. No one can maintain consistently that 

 Carrel's long-continued tissue cultures are normal, yet here we have 

 artificial conditions under which these vertebrate tissue cells con- 

 tinue, apparently indefinitely, to live and divide. Death of cells 

 occurs when the transfers are not made at appropriate intervals; 



