SYMBIOSIS AND THE BIOLOGY OF FOOD 271 



these are some highly noxious weeds, such as the rag- 

 weeds, cockle-bur, yellow dock, etc., against which the farmer 

 has to wage incessant warfare, since they are inimical to his 

 crops. Here we have plants, which — evidently in the absence 

 of due symbiotic productiveness — show highly redundant 

 rates of multiplication, simultaneously with mischievous, 

 " toxic " effects, which in this case extend to man. 



No satisfactory explanation has hitherto been forthcoming 

 for the fact that weeds are generally detrimental to crops. 

 We can now see that the antagonism is that between two 

 contrasting principles of life : the industrious and the impro- 

 vident ; the same antagonism, in fact, which is universally 

 observable throughout life. The inferior species labour not 

 for reciprocal or common, but for fundamentally anti-social, 

 purposes ; hence their productions do not support but rather 

 poison the strenuous community of life. 



The Nematodes furnish a good illustration of what I call a 

 divorce from Symbiosis, with its direful physiological and 

 biological effects. In the desert some Nematodes are actually 

 symbiotic and beneficial to the plants, but on cultivated 

 lands they very soon exhibit a striking weakness of symbiotic 

 disposition. Consider the following data communicated by 

 Dr. N. A. Cobb, of the United States Dep. of Agriculture 

 [Nature, April 4, 191 8). The genus Mononchus, which is of 

 world-wide distribution, has some species which are cosmo- 

 politan. Mononchus are regularly present in arable land of a 

 sandy or loamy nature, and sometimes occur in great numbers. 

 Dr. Cobb estimates that there were at least thirty million per 

 acre in the top six inches of a field of maize in New Jersey. 

 Most Mononchus are carnivorous. They have been found to 

 feed on protozoa, on rotifers, and on other Nematodes. 

 One cosmopolitan species was found in Florida feeding on 

 larvae of Heterodera radicicola, a serious root-pest, and it is 

 suggested that further investigation may reveal the possibility 

 of utilising Mononchus to reduce the enormous losses in crops 

 due to plant-infecting Nematodes. 



As our cultivated plants are generally over-fed and corre- 

 spondingly lacking in resistance, this provides the temptation 

 for predaceous excursions on the part of the Nematodes. First 

 they begin, as plant-murderers, to batten on the plants alone, 

 the lapse from Symbiosis becoming evident in redundant 

 multiplication and in abnormal evolution, which in turn offers 

 the temptation to other Nematodes to still more intense 

 depredation, in fact, to in-feeding. The result is so-called 

 obligatory carnivorism, or obligatory parasitism. The latter, 

 at least, avail towards keeping down the numbers of plant- 

 assassins, a check favourable to the plant and likewise to the 



