McKay. — On Diatoms at the Bay of Islands. 377 



Owing to the absence of intermediate forms connecting the 

 wide distinctions of the different genera, and other reasons, it 

 is evident that no evolutionary process has taken place. The 

 forms present are all well known to students of this branch of 

 natural science. But, while this is so, it is also true that the 

 theory that much of the deposit is derived from an older pre- 

 existing deposit of diatoms is not borne out by Mr. Maskell's 

 last examination. None the less, the results obtained are 

 exceedingly interesting. 



I may here point out that the conditions under which, 

 from first to last, the deposit accumulated were unvaryingly 

 the same, there being special provision in the regularity of the 

 intermittence of the supply- waters to this end. Yet, while 

 two genera, Navicula and Melosira, flourished in predominant 

 numbers throughout the whole period of the deposit, one, 

 Synedra, is found only in the first-formed foot of the deposit, 

 but in no great abundance, while three other genera show 

 several specimens at 9ft., are plentiful at 8ft., but in the 

 middle and higher part are absent or rare. 



The cause or causes of the disappearance of these forms is 

 a matter respecting which I do not feel warranted in express- 

 ing a decided opinion. Nor was this paper written with such 

 a purpose. As stated in the first paper on the subject, further 

 facts were needed in order that the subject might be fairly 

 discussed. It seems to me probable that variations of tem- 

 perature, not seasonal, but climatic, may account for the 

 gradual or comparatively sudden appearance and disappearance 

 of the four genera not found in all parts of the deposit. Any 

 other theory, it would seem, is either insufficient to account 

 for the facts, or demands too much by way of organic modifi- 

 cation or physical change. 



