Therefore every now and again some month must be left out or a month added. This necessity, at first not recognised, or not clearly so, is the chief cause of the above-mentioned disagreement in the reckoning of the months[876]. For when the counting is performed in accordance with the series only, it soon happens (apart from the climatic variations of the years already mentioned) that the months deviate from the natural phases from which they are named. The arguments in the dispute as to which month it really is are based on the condition of the phases of nature: the result is a correction of the counting, i. e. the months are pushed forwards or backwards according to circumstances, i. e. the month which should have followed is left out, or a month is added to the series. Thus an intercalation comes about without it being suspected what is really done. In general the whole process is not even so conscious as the desire for theoretical exactness has led me to represent in using the example of the Dakota. The series and the number of months were from the beginning unstable, and the natural conditions have brought it about that this characteristic has been preserved in at least one particular, viz. that in certain cases a month could be passed over. Let us, for the sake of clearness, take a fictitious example from Swedish conditions. As a rule the rye-harvest falls at the beginning of August, the oat-harvest at the end of August and beginning of September, the potato-harvest at the end of September. These occupations might very well be distributed among three months named after them. But a year would sometimes come in which the oat-harvest took place about at the interval between two moons, the rye-harvest at the beginning of the first moon, and the potato-harvest at the end of the second moon. There would therefore be no place for a month of the oat-harvest, it must simply be omitted. That this is the case among the primitive peoples is proved by the fact that many, in fact most, of them have a series of thirteen months of which one must according to circumstances be passed over in certain years.