The unicorns are multiplying, but if Andreessen Horowitz is right, the highflying venture capital community will have cause for concern when the inevitable thinning of the herd takes place.

The unicorns, of course, are the 60-odd technology start-ups that have taken investments valuing them at more than $1 billion each. The $50 billion valuation of Uber, the car-hailing service, has been held up as an illustration that Silicon Valley is in the middle of another bubble.