Eventually, around the 14th century, the cog reached its structural limits, and larger or more seaworthy vessels needed to be of a different type. This was the hulk, which already existed but was much less common than the cog. There is no evidence that hulks descended from cogs, but it is clear that a lot of technological ideas were adapted between the two types. The transition from cogs to hulks was not linear, according to some interpretations both vessels coexisted for many centuries but followed diverse lines of evolution.