This is bogus, and all you need to do to realize this is think about your own body - specifically, think about your spinal column. The mistake you've made is in assuming that the length of the spinal column and the length of the nerve are independent. Are you as tall as you were when you were 10 years old? No, probably not. What's my point? Occasionally in Statistics, we get these things called 'hidden variables' - it's where you get a correlation between two unconnected things - like chocolate consumption and road death tolls, but the correlation only occurs because of some other third variable - in this case population growth. In the case of the spinal colum, the only thing implied is that the size of the Vertebrae and the length of the nerve are controled by the same factor - in this case, as I understand it, the length of both are controlled by the same genes, and the growth of both is controled by the expression of the same proteins and enzymes, but there are (obviously) differences between the cells that become the bone, and the cells that become the nerve. So because the growth of both are linked to the same factors, you can not change one without changing the other, because both are controled by the same third thing. While this might seem like proof of an intelligent designer, by incorporating a 'fail safe' also consider that it is the 'simplest' way of arranging things. And while you might wonder why evolution would favour this over an alternative approach, stop and consider the implications of any alternative. There are two options available to us - dependant devlopment, or independant development of the vertebrae and nerves. I am suggesting that the information we have available shows that the development of both is dependant on some third factor (the Hox Genes), with the net result being the illusion of co-evolution, and this being the simplest way of doing things. The alternative is they are controled by independantly. Consider the implications of that. How long is a Giraffe with a spinal cord that's too short for its neck going to survive. The answer is, it probably isn't. It's probably going to die at an early age, due to the spinal cord being severed, or pulling the brain down into the spinal colum. How long is a Giraffe with a spinal cord that's too long for its neck going to survive for? Again, it probably isn't going to survive to sexual maturity, and it's probably going to die of some complication resulting from having your spinal cord exposed. Similar arguments, incidentally, can be made for the heart size, and blood pressure 'issues' as well. Giraffe bloodpressure (presumably) follows a normal distribution about some mean value (same for heart size). As neck length increases over time, those Giraffes that might ordinarly have been hypertensive, are going to find themselves being more successful with greater frequency than those with 'mean' blood pressure, and those giraffes with long necks, that would ordinarily have been hypotensive are probably going to be the least succesfull, and are probably going to find themselves prey pretty quickly. So we end up with a regime where Long Neck, Low blood pressure < Average Neck, Average Blood pressure < Long neck, Average blood pressure < Long neck, high blood pressure. The net effect being that evolutionary pressures, combined with physiological requirements are going to select for those Girrafes with long necks and high blood pressure. And please, you'll note that I only made two initial assumptions: 1. That Giraffe blood pressure exists accross a range of values. 2. That Giraffe neck length has increased. And from those two assumptions, we have effectively explained the co-evolution of girraffe blood pressure, and Girraffe neck length. Girraffes with neck lengths above the population mean, and blood pressure below the population mean probably weren't even able to stand up, and so probably became prey quickly. Giraffes with neck lengths and blood pressures around the population means, were as successful as every other giraffe. Giraffes with neck lengths around the population mean, but blood pressures above it, were no more or less successfull than the rest of the population. Giraffes with neck lengths above the population mean, but blood pressures around the population mean were slightly more successful, because they could reach more food, but may have had the occasional bought of giddyness. Giraffes with neck lengths and blood pressures above the population mean were the most successful, because they reach more food, and did not have the bouts of light headedness to deal with - this may even have given them an advantage over those from the previous groups, because they would be able to keep their necks erect more. No Magic, no mystery, just a trait that was already in the population giving a slight advantage.