This is a mono-green aggro deck with a focus on maximizing +1/+1 counters.
Two areas that mono-green is often lacking in are Removal and card draw. With Walking Ballista, Inscription of Abundance and Voracious Hydra it's got some very solid options. The downside of this removal suite is that none of it is particularly effective in those crucial first 2-3 rounds which is significant and should be taken into account in the fast-paced environment of Modern. As for card draw, I've tried a few options - including Tireless Tracker - but ultimately the best option thus far has proven to be Inspiring Call. Often dismissed as a 'win more' card, it's strength is it's versatility, allowing me to both protect my board state from a variety of wipes and to refill my hand, either of which alone makes it a worthwhile inclusion.
The good news is, if they haven't beaten you by turn 4, it's very unlikely that they will ever get the chance to do so. This is without a doubt one of the the most fun decks I've ever played - the things it can do are truly staggering sometimes - and while the fact that it can be a little slow out of the gate and lacks a reliable draw engine ultimately keeps it from being a top tier deck, it really does wreck people; it's not uncommon to have five or more creatures somewhere between 8/8 and 25/25 by rounds 5-7, usually with Trample as well.
Experiment One – chosen over Pelt Collector because to can evolve - technically better than Pelt Collector in that regard - as well as regenerate, making it more difficult to get rid of.
Swarm Shambler – comes in with its own counter without needing outside help, which is significant. It can grant itself additional counters (triggering Wildwood Scourge among other things) and punishes opponents who target your creatures by creating tokens. Not bad at all for a 1-drop.
Also considered: Pelt Collector
Avatar of the Resolute – can come in huge, has reach and trample and, in tandem with the evolve creatures, can cause Wildwood Scourge to absolutely blow up in a single turn.
Gyre Sage – evolve alone is super strong in this deck, but the additional mana can dramatically accelerate your turns in the midgame. When in doubt, drop the Sage first so that you can use it's mana as soon as possible but reserve it's use until after you've exhausted the rest of your mana.
Duskshell Crawler – drops a counter wherever you wish (usually on itself), grants all your creatures trample (substantially reducing the need for Rancor or Pelt Collector) and has a significantly offset power and toughness, making it more likely to trigger evolve creatures.
Wildwood Scourge – I'm including it here because I most often play it as a two-drop, relying on everything else to get it big, but it also serves as a solid mana sink for later in the game as well.
Also considered: Strangleroot Geist, Merfolk Branchwalker, Scavenging Ooze, Wildborn Preserver
Yorvo, Lord of Garenbrig – comes in with its own tokens and grows substantially with every new creature, in turn giving every new creature a chance to trigger Wildwood Scourge twice.
There are two excellent options for an additional three-drop creature (Tireless Tracker and Renegade Krasis, and I went back and forth over them before finally settling on something else entirely for these slots: Inspiring Call.
Also considered: Tireless Tracker, Renegade Krasis, Champion of Lambholt, Predator Ooze, Jadelight Ranger
Walking Ballista – comes in with its own counters, can add its own counters, serves as a superb mana sink and offers direct damage, potentially in droves. No brainer here.
Voracious Hydra – can come in huge and has a built in 'fight' function for additional removal. Another excellent mid-game mana sink.
Also considered: Stonecoil Serpent, Mistcutter Hydra, Hungering Hydra, Verdurous Gearhulk, Forgotten Ancient
Hardened Scales - No-brainer.
Also considered: The Ozolith, The Great Henge, Nissa, Voice of Zendikar, Rancor
Inspiring Call - card draw AND force protection in one. Hard to beat that.
Inscription of Abundance - again, versatility is the watchword. All three options are useful ones individually, but I find myself using the kicker for all three more often than not.
Also considered: Aspect of Hydra, Mutant's Prey, Smell Fear, Solidarity of Heroes