NO. 4 ALLEN : PLANKTON DIATOMS 55 



Other forms. Of course, the evidence is too little to support an explana- 

 tion of the peculiarities involved in these occurrences, but there is 

 a suggestion that the Gulf may be more favorable to this species at 

 certain times and places. Possibly a different form of statement is 

 preferable, i.e., that this species may have been better able than 

 others to meet the obligatory conditions. A good explanation of the 

 peculiarities of occurrence of this form there and then would surely 

 contribute largely to solution of the perennial problem of why abun- 

 dance of plankton diatoms increases at a particular time and of why 

 any certain species leads in abundance at that particular time. For 

 the catch in mid-Gulf one might be content to suppose that the 

 sample merely chanced to be taken from a center of propagation of 

 this species. For the catch taken under much different conditions in 

 the narrow, shallow bay, this supposition may not appear so accept- 

 able, especially when one considers the possibility of explaining both 

 of them in the same way. My own belief is that causes of change In 

 abundance or in lead of abundance in nature do not operate uniform- 

 ly or steadily, and that the results of their operation are not predict- 

 able in detail. In a case like this, the population in mid-Gulf may 

 have been preponderant because of its physiological readiness to take 

 advantage of the first favorable opportunity, while the population of 

 the bay may have been holding Its leading place merely because of 

 greater tolerance, or greater capacity for endurance for bay condi- 

 tions. But It Is still true that in both cases T. heteromorpha may have 

 had more representatives on the ground ready to meet prosperity 

 when it approached. It seems probable that the Gulf offers good 

 opportunities for solving certain parts of the general problem of oc- 

 currence of abundance and changes In abundance. 



Condition of Specimens 



Following the Scripps Institution practice of the last several years, 

 records were kept of numbers In poor and in good condition. This 

 series In the Gulf of California was remarkable for the relatively 

 small numbers of specimens in poor condition. In some series studied 

 at the Institution many catches (some very large) have shown 

 much more than half of the specimens to be In poor condition (the 

 frustules empty or the contents disintegrating), but In this series no 

 catch of significant size showed as many as ten per cent of the speci- 



