AMONG UNICELLULAR ORGANISMS 145 



conditions facilitate asexual reproduction, but are inimical 

 to conjugation. Unfavourable conditions which exhaust 

 the vitality of the organism facilitate conjugation and 

 sexual reproduction until carried beyond a certain point, 

 when conjugation becomes increasingly difficult. On the 

 evolutional theory we must expect the higher organisms to 

 be governed by the same biological laws as the primitive 

 forms from which they were originally derived. And 

 the evidence that such is the case is overwhelming. 

 Therefore it becomes a question of the method by which 

 the principle is applied among the higher organisms. 

 It is only as a law governing the union of sperm cell 

 and ovum that the principle can apply to all forms of 

 sexual reproduction, from the most primitive unicellular 

 organisms, through all grades of plants and animals up 

 to man. No special physiological adaptation connected 

 with ovulation or any other of the sequence of reproductive 

 phenomena would suffice to meet all the various methods 

 of fertilisation throughout the plant and animal kingdoms. 

 On the other hand, the sperm cell and ovum, being 

 merely specialised unicellular organisms, will, we may 

 be confident, be governed by the same laws as the primi- 

 tive cells from which they were derived, and these laws, 

 as we have seen, actually produce the same results as 

 those manifested in the plant and animal kingdoms. 

 It is necessary, therefore, to assume that the condition of 

 the ovum at the time of ovulation reflects the condition 

 of the organism as a whole. But this can only be affected 

 through the agency of the nervous system, or, in the 

 case of plants, the hypothetical factor which we must 

 assume to take the place of the nervous system in animals. 

 The nervous force is probably analogous in character 

 with that which animates living cells, even though these 

 possess no nervous system, as it is pre-eminently the 

 life force. It can and does influence every cell through- 



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